As a general rule, the less the valve is shrouded the better. But i have found, that if you get real crazy with it, it can really mess up the way the air travel through the port. I only have experience with the J,X, and early emission 360 heads.
That is why i asked, as this is the only ones that i could give you more then theory on.
I guess I'm not answering you question but i don't have any cold hard fact on this. My guess is around .020 on valve to valve and as far as shrouding goes i would say .020 at .100 lift and continuously enlarging as the lift goes up.
Unfortunately you can't do that as the head gasket surface is in the way.(were talking intake valve here)
