If I remove a bayonet-mount lens from an SLR and measure from the front mounting plane of the camera where the lens was removed to the ground glass, I assume this is the registration distance. Does film thickness need to be subtracted to get a 'real number'?

If so, is 0.13 mm for 35mm film right, and does anyone have a number for 120 film? I read 0.2 mm somewhere, but don't know if the film is thicker or if this DOES include the paper backer.

While on the topic, if film thickness has to be subtracted from the 'empty' registration distance, on a sheet film camera with removeable ground glass, I plan to have attached spacer on the glass, and if the film 'counts', I shoudl make the spacer smaller than the ANSI filmholder distance?

Thanks