A rule of thumb I use, approx:
410mm for 51mm - 63mm travel.
420mm for 63mm - 80mm travel.
440mm for 80mm - 100mm travel.
The thing with bontragers is that they are designed around a fork with lower rake then normal, 1.25” (31,7mm) - it might be hard to find one if it’s not a bonty, most rigid...