An easy way to do the calculations is to use Parker’s O-ring handbook. There is a section that will tell you the range of forces/liner inch at a certain durometer and o-ring width. That will give you a starting point that can be translated into overmolded parts. In practice, 30-50 is very soft and probably right for IP67, 70-90 is better for greater protection. At high pressures (spray or depth), you will need other tricks.
Balance compression force with number of fasteners and the stiffness of the part.
In general, I would say the Parker book is a conservative approch, but right order of magnitude.
Figure 2.4 in the parker book here: https://www.parker.com/literature/ORD%205700%20Parker_O-Ring_Handbook.pdf