You know that I have been teaching water care for 12 years in classes and what I have found is a lot of messed up spas because of good old "Miss Information". To be politically correct now days it is "Ms. Information".   :D

Starting with the Calcium numbers:  The purpose of calcium in a spa is to help buffer the pH and make the water less of a solvent. 
If you take distilled water with an eyedropper and spa water with 200 PPM and put a drop of each on a piece of stainless steel, the distilled water will start to rust.  The calcium water will leave deposits of calcium as it drys but no or almost no rust.

In a swimming pool the calcium is to stop the water from dissolving the swimming pool walls, plaster and concrete.  Concrete is held together with calcium and water will dissolve it. So, put calcium in the pool water to slow down the destruction of your pool. The numbers for a concrete pool are over 350 PPM in some books on pool care.

Some professionals don't know this, and the spout off numbers that are not appropriate for the type of materials the "pool" is made from.  If you have a vinyl liner pool you need to keep the calcium to about 200 PPM like a spa that is made from plastic.

If you have too high of calcium, you can easily ruin an electric heater, by coating it with calcium, simply by screwing up the pH to above 8.2 or higher and not using a sequestering agent, like "Spa Defender" or "Stain and Scale". This is why we only aim for 180 to 140 PPM of calcium

One of the things you should absolutely never do is use "pH Perfect" or any of the liquid pH stabilizers, because I have seen too many ruined jets and heaters, mazzie injectors plugged and filters ruined.  Use pH Magic ONLY!!!  Or don't use any pH stabilizer at all, unless you like to meet your repair person regularly.

Use a boric acid type of product, if you are having trouble with pH,  instead to help buffer the pH.  I like the pH Magic product and have not seen any ill effects from it other than raising the TDS (total dissolved solids).

As far as the Total Alkalinity you need to listen to me. I have gotten people to stop wasting a lot of pH products in the "fight" to get the "numbers" right. 

First of all pH is primary.   Repeat that a few times until it "soaks in".  TA, total alkalinity is secondary and is the assistant to the pH.  Repeat that a few times until it soaks in.

If you adjust the Total Alkalinity to some fixed number, like it is "etched in stone"  and then fight to get the pH down, you are wasting products (money) and increasing the TDS for no reason other than to waste chemicals and shorten the time between draining and refilling.

The Sodium Hydrogen Carbonate is a way to "cloak" what is in Total Alkalinity increaser.  It is baking soda, sodium bicarbonate, and that is all it is.  We sell it to our customers in 5LB of an off brand to make it less per pound than the grocery store.  Don't listen to the advertising video put out by the chemical companies: "Be sure to only use our brand of chemicals so you won't screw up your spa." That is the implication. But you must know what the chemical is that you are buying so you don't use pool chlorine in a spa or spa shock as if it was chlorine and such.

For Spas:
When you start  out testing the water start with the recommended numbers on the TA, then adjust to your situation.  The idea is to control the pH.  (repeat twelve times).   We use the TA to control the pH, not the other way around.  Anybody who tells you to adjust the TA first to a fixed number is wrong, totally wrong and is causing too much work and too many wasted products.

The idea it to get through a week without having to test and adjust the water for TA and pH.  If your spa has a regular use of three to four times a week it is easy to get this under control.  If it is used 5 to 6 times a week you can still do it, just a little harder to do.  If the use is consistent it is a lot easier to get the water under control.

If the pH goes up to above 7.8 to 8.0 with the TA of 100, then next time try 90PPM.  But you never want the pH to fall below 7.0.  If you can get the pH to stay about 7.6 on a weekly basis then you would raise the TA back to 90 and lower the pH to 7.2 to 7.4.  The less chemicals used the better.  [b]Adjust the TA to work with your results.[/b]
I have customers who use a TA of 40 PPM and are able to keep the pH in tight range of 7.2 to 7.4 and only have to put in a small amount of TA up and not have to adjust for the pH at all.  That is only for their spa and their conditions of use.

I have others who use a TA of 80  to 90 and no problems.  Some need to go above that if they are using a lot of shock with tons of bathers.

It is the amounts of acids put in the water that lowers the pH. The TA in a spa will pull the pH back up and keep it from falling below 7.0.  This is just a way of describing the action.  If the TA is too low the pH will fall into a drop off condition and fall quickly.  If the TA is too high the pH will keep going up and you will be wasting the pH Down product to fight with an improper TA number for your spa.

Now, this is the big secrete to getting the TA and pH to the numbers you have worked out.  I learned this method from an "old timer" who used to adjust pools and spas professionally.

Since the TA raises the pH and the pH down lowers the TA, how in the heck do you get them both correct if you put them in at different times.  If you get the TA "perfect" you have just screwed up the pH. If you get the pH "perfect" you have just screwed up the TA.

The trick is to figure out how to over dose in both directions at the same time.  If you know that the "pH Down" is going to lower the TA, then put in too much TA up (a controlled overdose). If you know that the TA up is going to raise the pH, the put in too much pH down.

This only works if the spa is in a controlled range. With the acid demand system no more than three drops of acid demand and no more that 30 PPM of needed to raise on the TA.

Using a Tablespoon as if it was 1/2 ounce, use the charts and pretend that it is 1/2 ounce.  It is really closer to .7 ounces to .75 of Sodium Bisulfate. 
If the chart says to put in .98 ounces of pH down, put in two table spoons (about 1.5 ounces) .  If the chart says to put in 2.7 ounces of TA up, put in 5 tablespoons.  When you retest it is amazing how close it is.

I had a fellow in one of my classes who has owned a spa for over 10 years and when I told him that I could get the spa water in balance with this method, he said: "impossible".  There is no way to get the pH at 7.4 and the TA to 100 (this was a bromine class).  I always had a floor model that was needing to be adjusted for the example spa.  After I adjusted the spa water and finished the class, I told the fellow to come and test the water, but he would have to wait about an hour at least before the water started to settle in.

I don't think there is a way to describe the look on his face when he tested the water.   :D

I just made his life a heck of a lot easier and cut his pH balance chemicals in half.

When in doubt ask someone who does it professionally how they do it.  And if they tell you that it is impossible, then you can teach them how to do it.

One of the other issues with the TA is the myth that if you want to lower the TA you dump the acid in one spot in the pool/spa, but don't sprinkle it around.  It makes absolutely no difference at all.  It is a myth. Acid is acid and it has the same effect no matter how you pour it in.

Oh! Yea.  I only recommend using a Taylor Test kit with the Acid and Base demand tests.

The ideal is to have the pH raise a little while the TA drops a little.  That way you are not "in danger" of the pH falling at the end of the week.

I hope this clears up the issues with pH and Total Alkalinity.  Enjoy your spa and keep the water balances and you will not see the repair guy very often, and if you use the Ions or Eco One, you will make your spa last twice as long as with bromine or strictly chlorine.