For any lens to focus properly, the critical factor is the register (lens flange to film plane distance). The relevant values are:
Originally Posted by chrism
Leica screw 28.8 mm
Leica M 27.95 mm
It therefore follows that an adapter for a LTM lens on a Leica M body must be 0.85 mm thick. If you have a micrometer and can check your adapters, you should find they are OK in this respect. From an optical point of view, it does not matter at all where the witness mark for the lens focus is positioned - cosmetically it looks neatest if it is in the 12 o'clock position, but the lens will work fine even if this is not the case. Of course rectangular lens hoods need to be positioned squarely and will have to be adjusted if the lens position is off. In general, you should find that Voigtländer adapters are better made.