when setting points (on anything) i jog the engine (or entire vehicle if easier) so that the points are open at the highest point on any lobe on the distributor. set the point gap with the correct feeler gauge and adjust as required (generally a turn of a screw). you want to set the gap when the points are breaking on the highest spot on the distributor lobe.
top dead center / before top dead center only applies when setting engine timing, and not point gap. i'm not the least bit familiar with that tractor, but you should be able to find that information all over the internet. if not, call any deer dealer and the service or parts desk can probably tell you. i did, however, recently replace the points and set the timing on a 1950's john deere 420 crawler tractor, though that's a different animal.