if i do three boils on the bark and add acid during each boil, how much hydrochloric acid should i add to 175g of mhrb each boil?

5 mL for every 1000 mL I think. Check with pH papers, you should be left at pH 4.

HCL is very powerful, a few drops will be good.

if you do a STB tek you even don't need to use acid, nor boil, give it a try.

I have to disagree, A/B are superior to STB in many ways...time is not one of them, but a clean product, not having to find ways to get your naphtha out of the sludge is another, and not having to use 1/2 a container of lye is a third, out of many.

To the OP: HCL is a great acid to use, but vinegar is safer, cheaper, and works just as well. Using HCL does not seem to increase yields in my extractions. Phosphoric, however, for some reason seems to work better than either vinegar or HCL, still I tend to use vinegar.

1 cup vinegar to 3 cups water for the first boil, 1/2 cup vinegar to 3 cups water on 2nd + 3rd, and straight water on the 4th. This is how I usually do it. It doesn't take much NaOH to get this to a pH of 12.5

Well you can always do a recrystallization when doing a STB, in order to get pure spice.

^ Very true.

STB isn't a bad method, many people use it with great success. I've always had issues with mine, and just prefer the A/B. Find what works best for you of course!