Using M42 mount lenses on 60D - maximum backfocus allowable?

I have an old Vivitar 20mm f/3.8 manual lens in M42 mount. It sticks out behind the M42 to EOS adapter. Does anyone know what the maximum backfocus (mm) is that allows sufficient clearance for the 60D mirror? The interesting part about the Vivitar is that it has 1:3 magnification at minimum focus, quite close for a ultra-wide angle lens, and it is short enough to take a 7 mm M42 extension ring and still have (barely) ability to focus to 1.5 cm in front of the front element (for  approx. 1:1.3 magnification).