I am attempting to build a 12 sided monopole section.

The dimensions are as follows

40" diameter at the bottom

31.73" diameter at top

45'-4 1/2" tall

.375" thick

I've drawn the bottom sketch, extruded it to length and applied the Thin Feature.

But I can't for the life of me figure out how to calculate the draft angle to give me the 31.73" diameter at the top.

How do I calculate this angle, or am I using the wrong feature to create this?

Thanks in advance!