One extra thing also - different Leica M cameras have different accuracy, and it depends also from lens you use.
M6 (0.72 vinder version) is very accurate on 50mm on close focus, but very wrong on infinity showing only around 85% of what you get on film. For 35mm lenses M6 is the best - biggest amount of viewfinder is in use, and very precise on close and infinity.
M3 is very good on infinity, but on close focus you get less on film than in viewfinder.
For other M bodies I can't say anything since I don't have them. I have made this conclusions by testing them in parallel with nikon F3 who is for me prefect accurate machine . Maybe someone else had made some testing for M2, M4? I guess M7 and MP are the same as my M6.