The issue for power brakes is making sure they function "ALL" the time. That means Panic Stops, mountain driving and long steep downhill grades. That said I would like to see a minimum of 12hg at idle on the gauge for safe operation under all conditions. 14hg would be better. That equates to around 60* of cam overlap in a sbc. This depends on the motor and how efficiently it runs but it is a good ball park figure in overlap to shoot for. Less overlap, more Hg. More overlap and the Hg falls off more quickly. My 383 has 58* of cam overlap and I have 13 - 14hg at 650 - 700 RPM idle. Jack