Anecdotally, I've been seeing about .2 drop in ph between preboil and post. For example, a 5.2 is usual 5.0 post boil. Until a couple days ago with a cream stout. 5.2 ph preboil, then added a pound of lactose at 10 minutes. Guess what my post boil ph was. I'll give you a hint. It was 5.2