Ditributor Advance - Looking for knowledge

The way i did it was to do a ratio between the length vs degrees of the original plate and solved for the length they needed to be to have the amount of advance I wanted.

A 15R plate=30 degrees of crank advance. I wanted a "10R" plate to have only 20 degrees of mechanical advance.

so the formula goes as follows (original slot length)/(original advance)=(new slot length)/(new advance)

solve for new slot length:

new slot length=(original advance/new advance)*original slot length.

Worked perfectly for me.