What is taper?
The fetched Omni taper page treats taper as a conical frustum: one end has a larger diameter and the other a smaller diameter, with a gradual reduction over the taper length.
How to calculate taper per foot
TPI = (Dl - Ds) / Tl
TPF = 12 x TPI
theta = tan^-1(TPI / 2)
Source example
Omni's worked example uses major and minor diameters of 12 and 6 inches with a taper angle of 80.5 degrees. That gives a taper per inch of about 11.952 and a taper length of about 0.502 inches.