I've never used a Bigsby but I'd simply look at a few photos of Bigsby equipped SGs to get an idea. Looks to be about 2"/50mm back from the bridge. I can see some variations in the positions, so it's not critical. What you need to think about is the string break angle. You don't want the Bigsby so close that the strings are pulled onto the rear of the bridge. Or so far away that the break angle is too shallow so there's not much downforce holding the strings in the saddle slots.

I'd set your bridge up to the height you'd expect it to be once the guitar is set up (run a straight edge along the neck and have the saddles about 3mm higher than the spot where the edge meets the bridge). Then set the Bigsby about 50mm behind the bridge and look at the angle the strings will take from underneath the roller bar to the saddles. If there's any risk of the strings touching the rear of the bridge, then move the Bigsby back a bit until you know you won't have issues.