I tested the rule of thumb out with a few small mashes only changing the percent of acid malt added, same grain & water for the mashes and got a similar trend to the Weyermann rule-of-thumb. I'd also just use 0.1 pH change per each % of 2% Acid Malt added to the mash.
pH meter was a Hanna pHep5 with fresh 7.01 & 4.01 buffers.
