I'm planning to brew a robust porter as the first batch with some locally grown and malted barley that the maltster informs me is high in beta glucans. I'm planning a 20 minute beta glucan rest at 113F then a 60 minute sacc rest at 152F. For this beer I'd normally build my water to target a mash pH of around 5.4 but I'm trying to figure out how address the fact that optimal beta glucan rest pH is around 4.5 - 5.0 pH. I understand the factors affecting pH in a typical single infusion mash but I cannot get my head around this situation. I use Brun Water and I'm thinking maybe I should start by working up what it would look like using my typical water profile for a single infusion mash, then back the mash water down to the amount that will go in for the beta glucan rest infusion and see if that puts me in the ballpark of an appropriate pH for the step. The problem I see with this is that I know a mash step doesn't reach its pH immediately and I imagine Brun Water assumes a mash that's had time to reach equilibrium. I'm stumped. Can anybody advise me?