For the linear pressure-overclosure option, you can get a feeling for the value by thinking of it as a layer of elastic material (blue in attached picture) between the two parts. Imagine it has a thickness and Young’s modulus that will cause negligible relative displacement (eg. a thin steel shim between two large steel parts) and calculate the stiffness as that layer’s Young’s modulus / thickness.
Perhaps a simpler way to think of it - how much extra thickness can you add to one of the two parts without significantly altering its stiffness in the direction normal to the contact surface? Calculate the contact stiffness as E/t where E is the part’s E and t is the extra thickness.
The recommendation of
50*E
isn’t generally valid because the appropriate value also depends on the scale of the objects. Also, the units aren’t consistent. I recommend to start with
10*E/L
where L is some characteristic length that I can’t quite define, but maybe the thickness of the parts. Then increase it by a factor of 100 or 1000 if you need to. Starting too low rather than too high saves frustration because if it’s wrong, it still gives a solution quickly and you can see the problem of penetration whereas starting too high can leave you waiting hours for the solver to eventually fail with no clue about what went wrong.