Originally Posted by ajdelange
What you really should do is is obtain a pH meter...
I think this is exactly what I'll do. If I'm going to start working with my water chemistry, I should do it right. That means taking measurements instead of making estimates. Now I just need to find a reliable and accurate pH meter that won't cost an arm and a leg.
As it happens, I ended up changing my plans at the last minute and brewed an IPA instead of the porter (using the primer's recommendation for British beers). Once I get my pH meter, I think I'll do a small stove-top mini-mash experiment on the effects on mash/sparge pH of adding dark grain @ vorlauf.
Thanks again to both of you for the feedback.