The rule of thumb for static face seals is to design around a nominal 20-30% compression with a worst case including tolerance stackup of >10%. Reciprocating seals on shafts (i.e. buttons) are typically designed around 15% compression with a minimum compression >10%. It varies somewhat for different materials.
I measured my 5D housing since it has a traditional rectangular o-ring groove. The vacuum system is on my 5D3 housing which uses a dovetail groove. The calculations are a bit tricker for the dovetail, but I pump my housing down by 10inHg = 4.9psi. The cross sectional area of the rear door is roughly 50 in^2, so the vacuum is applying an additional 250lbs of force. Jean said that they set the latches to apply 45lbs, so an additional 250lb should be safe ;-)