As waterbear noted, knowing an accurate amount of acid or base to add to move to a specific pH is best determined through an acid/base demand test such as found in the Taylor K-2006 test kit. However, The Pool Calculator does a decent job estimating the amount and accounts for the various buffer systems in the water including carbonates (TA), borates and CYA. However, at the time the calculator was made the accurate calculations in my spreadsheet were too complicated to put into the calculator so Jason created tables/formulas that approximated the effects. Since that time, I've found simpler formulas for the accurate calculation and this is discussed in the thread pH Buffer Capacity. Unfortunately, Jason sold The Pool Calculator to pSIFlow Technology Inc. and they don't seem to be doing any maintenance on it.