When getting a machinist level, I would advise to check that the V-shaped groove on the bottom is not too wide (if the level has one). If it is too wide and you are using the machinist level on top of a carpenter's level, the machinist level will most likely not sit accurately on top of the longer level because the V-shaped groove's edges go past the edges of the carpenter's level. And it will not give an accurate reading. The machinist level will wobble sideways and if you are pounding the slate wedges in, the machinist level almost surely will drop to the slate.
I don't have a Starrett (I use a 0.02mm/m level that has no markings as to make or model) but from what I have seem in pictures, those Starrets that have a V-groove don't have the problem detailed above.
I don't have a Starrett (I use a 0.02mm/m level that has no markings as to make or model) but from what I have seem in pictures, those Starrets that have a V-groove don't have the problem detailed above.