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Boric Acid, questions

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I plan to use boric acid on my next spa refill.  It is a plaster/tile spa that I estimate to be between 300 and 350 gallons.

The figure I found says to add .0457 pounds per 10 ppm per 100 gallons.  So, for 350 gallons I would need about .8 pounds to achieve 50 ppm.  The recommended 'range' I believe is 30-50 ppm...so, if my spa is 'between 300 and 350 gallon', adding 2/3 of a pound (302.4 grams) would still land me at the 40-50 ppm range.  Is my math correct here?

I found a 340 gram package of pure 'cosmetic grade' boric acid on ebay...which sounds like a pretty ideal size.  Does this sound like an acceptable source of boric acid?  Also, am I correct that it only needs to be added once, even if I keep the spa water for 4-6 months?  If it dissipates to a small extent, and/or if I lose some water to splash out...I figured I could use 300 grams upon refill and then maybe add the remaining 40 grams from the package after a few months.

One last question that I have is how/when to add the boric acid after refilling the spa.  I have a pretty high TA and it takes me that first day or so of gradually adding acid and aerating to get the PH to stabilize.  I also add calcium since I have a plaster/tile spa and the well water runs through a water softener.  So, can I add the boric acid AFTER I've finished adding all of the acid and the calcium?...or, would it be better to add at a different stage?  And, am I right to guess that I can go ahead and just add all 300 grams of the boric acid at once, and that it won't have any drastic immediate effects on the PH or TA?

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The online pool calculator should be able to calculate how much boric acid you need to get into the range you are looking for.  For my 362 gal tub, I add 12 fl oz of boric acid to get me in the 40ppm zone.

The general guidance I have found for adding it is to wait until you get your ph where you want it to be, add half of the boric acid and check pH again and then add the remainder if pH was still ok.

Not sure about replacement over time, but it stands to reason it would drop with carry out and absent in replacement water.  Hard to say unless you have a test for it.  The product I use has a nice smell so we add another oz of it every month or two after fill.  We change the water on a 4 month cycle.

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I appreciate the replies and welcome any further feedback.  

I was looking for a good place to order the boric acid from and found 'bulkapothecary.com'. They have 1 lb quantities for $5.40, so I ordered 2 lb...which should cover me for the next 3 spa refills.  With shipping, the total was around $22.  If using the boric acid allows me to add dry acid less frequently, then that will probably offset the cost somewhat (or perhaps entirely)...either way, the less frequent maintenance makes it worth the price.

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I buy mine from Duda Energy in Georgia. You can also get their products on Amazon. Their website is under DudaDiesel. They sell supplies for making biodiesel fuel and, IMHO, have very good prices on boric acid.

I normally buy the granular form instead of the powdered since it's easier to work with, IMHO.

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