My 5-C's have a Jarno taper, 0.60"/ft.
This taper was designed to involve no math to calculate. The large end
is the taper no. in 1/8th's, the small end is the number in 1/10th's
and the length is the taper number divided by 2. The taper in the c-5
is a truncated #13.
Hope this helps, Bugs