An invaluable image I refer to all the time, this guide displays the framelines for different focal length lenses for rangefinder Leica M6 cameras with 0.58x, 0.72x and 0.85x viewfinder magnifications.

Leica M body framelines for 0.58X, 0.72X AND 0.85 viewfinders

Leica M body framelines for 0.58X, 0.72X AND 0.85 viewfinders