I'm in the same boat. I'm trying to grasp how certain changes affect trail, handling, etc. but there is not an online calculator that takes account for axle-to-crown changes. When I get some time I'll probably do the trig, but in the meantime I see conflicting information most everywhere I go.

But I have seen, in multiple places, that if you add 10mm to the a-to-c, it will result in about 1/2 degree less of head angle.