Last time I set up some new shackle mounts I just sketched the leafs up in autocad at the various combinations (free state, loaded state, flat, expected max compression) and based my shackle on that. Worked great. I used autocad at the time, but any cad package (like autocad LT which you should be able to download free) should be enough to do it.
The thing about shackle angle, is that both the angle and the length play a part in the spring rate. So from that perspective, I think it's worth 'doing the math' to figure it out. But that's just me. I tend to overthink things (and then screw it up anyway).
Aren't you running YJ springs? Maybe a YJ frame would be a good starting point. (although the spacing here is about 43.78" front and 44.30" rear - doesn't sound right for a 42" spring).