What is the fit between case and caseback for a snap-on type

I tried to find a good answer but couldn't.

As an example, If I have a watch with a lip on the case that is 30.00mm diameter, what diameter should the caseback be machined to to give a good "snap-on" fit ? And are the contacting surfaces parallel to each other, and also perpendicular (90°) to the plane of the case, or is there an incline present on either of the mating surfaces.

I ask as I need to make a snap back for a watch I bought that has the wrong one.

Thanks Archer.

It seems to be fitted with a case back, but obviously the wrong one as it's a stainless steel watch and the caseback is marked up as gold-plated. I could measure this caseback and open up a steel snap-back to the same size, but would like to know really what is correct and original for the case. I'll have a play when I finally get the watch in my hands next weekend.

Also might be nice to have a display back, but not sure if this can be done with snap-backs due to the stiffness and loading when installed and removed.

I haven't seen it yet, but it will almost certainly be a case with a lip and a "female" case back. I guess the best option will be to find a donor caseback which is slightly undersize, and machine it out to give the right fit. Would have been nice to have a display back, as at least that gives an excuse of not having the right caseback...

I was assuming that the caseback is base-metal rather than stainless, but if it turns out to be stainless then yes I can turn off the text.