It’s easier to find the correct bridge position if you can string it up first, and run the strings over the bridge to check the alignment fown the neck. With a Bigsby instead of a stop tailpiece, that’s not so easy, but to hold the strings behind the bridge, I’ve seen people use a cheap trapeze tailpiece simply fixed to the rear strap button position, with the bridge raised up to a fretboard-clearing height on a suitable thickness of cardboard. You still need to get the bridge angle correct, but you can certainly mark the side-to-side position with ease, rather than try and work out a centreline.