I purchased an inexpensive angle finder from Harbor freight, it looks like the only way to find the correct angle on the Kingpin is to remove the backing plate from the axle, is that the way to do it?
The machined surface on the top and bottom of the kingpin bores is a little restricted by the backing plates, but if you had a straight steel bar you could hold that on the flat surface there and use your magnetic angle finder.
[on edit] Added picture.
