The answer to this question will vary depending upon the individual set up. Things that will affect this primarily are....are you a leaner or sitter, do you have a platform, what height are you attaching your tether.
I have a Kestrel, I use a platform, attach my tether around chin height and prefer a 24-25" bridge. I almost exclusively am a leaner.
If you having difficulty finding what length bridge you need, make an adjustable bridge and play with different settings. You will eventually find out what works best for you.
Good luck.