I shoot a lot of HP5+, mostly in 4x5, here is what I do:
I set my meter to 400 (box speed).
I place the darkest area where I want to retain detail in zone IV
I expose two sheets identically (both sides of a film holder)
I develop one sheet normally (I use Ilfotec HC at 1+31 for 6.5 mins)
I inspect the developed sheet to make sure that I have detail in the shadows, and in the highlights, if not, I adjust development for the second sheet.
For roll film, with normal contrast subjects, that would be similar to rating the film at 200.