Hi Dave,
Some years ago, I worked out that 36 degrees BTDC on postwar Douglases equals .276". On Mark machines, the mag gear will go through the hole in the back of the timing chest, so you can set it up before fitting the mag. I would suggest making sure the taper is clean and dry, then fit the wheel with a spring washer under the retaining nut. Just lightly tighten the nut so that the wheel drags on the taper, fit the mag and turn the armature until it is correctly timed. Carefully remove the mag, give the wheel a sharp, light tap to seat it on the taper, remove the nut, swap the spring washer for a plain one, then fully tighten the nut and refit the mag. Remember that Mark mags are set up with tight wire advance (unlike most British bikes). When messing about with mags on a Mark, it pays to remove the 2 HT pickups so that you can fit the 2 ¼BSF bolts after the mag is lined up.
Regards,
Eddie.