I have 3 surface plates, and here's what I'm doing with them:
- an 11"X 9"X2.5" (unknown grade) from China - used only as a bench plate, for lapping only. supported by bench. stored in a drawer.
- a 12"X18"X3", (B grade) - just unmounted it from its stand, because I'm reusing the stand. It will be mounted on 3 points, just because it's easier.
- a newly acquired 18" X 24" Mitutoyo A grade plate. see below...
... first a short story: I have a friend that bought a Mitutoyo AA grade surface plate in the 36 X 48 X 5 inch size - and mounted it using the 4 corners... After 36 years, it has dropped about .006 in the centre. It is now worse than a shop grade plate. I wouldn't take it if he gave it to me.
So being a little OCD and/or anal-retentitive (yes it does have a hyphen BTW), and because I'm not from Standridge Granite I am mounting my new surface plate on a rolling Tool and Die cart, also recently acquired, which is made of 10 gauge pressed steel. The plate will be mounted using 4 Bessel points with a pivot...
The best advice I can give to you is that if you are mounting it on a non-movable table, make the feet of the table adjustable so ti is properly supported( so the table won't twist) . No use twisting your surface plate for the lack of 1$ worth of leveling bolts. If it is on a moving base, the your plate's sub-base must be overly rigid, so as to support the plate properly. No matter what, the plate has to ber very rigidly supported to maintain calibration.
3 or 4? your choice - if your manufacturer won't commit, I'd use 4. If you go with a Standridge 24 X 36, they explicitly say use 3 because they calibrate them that way. I hope this is more helpful than confusing,...