I won't say 38* max is a hard fast rule, but 45 total mech (i.e. not vac) is really high for your application. I have run up to 42* on pump premium at 10.2:1 compression but I'm at high altitude and can get away with it.

I think for your application 36* total, probably around 12* on the crank is a very good starting point and you can tweak from there.

-Chris