First of all, the bridal fly is forward of where you are when sailing downwind -- and sailing downwind is why you use a bridal fly, right?
So, forget the bridal fly and tie a piece of 8-TrackTape (easy to find cheap at any yard sale) to the sidestay. That way you can actually tell if it is 90-degrees, aft of that, or forward of that.Much easier to read when it is reading from where you are sitting. Much easier to read and significantly less expensive.
Interestingly, When I fly a spinnaker I have a tape on the end of the pole, one on the bridal fly and one on the side stay. Guess what! They all read a different apparent wind direction.
The one at the end of the pole is almost 90-degrees, the one on the bridal is more aft, and the one on the sidestay looks as if I am on a close reach.
Use it all.
Rick