What are you trying to accomplish? If it is to go fast comfortably, the goal is to find the best compromise between aero, power and comfort. More back angle is faster wind-wise. After a point, it starts cutting down on both comfort and power. Your fitness and flexibility are huge here. (All this said - seat position rules! And is so individual I am not going to even start to tell you what to do.)

I don't think there are magic numbers. I am skinny, long and light. I need a near flat back to go either fast or upwind. 45 degrees is me on the tops for a long hill climb or just cruising. (I ride stems where if I actually sit up, my hands are 10' from the handlebars.) I have also found that, for me, the magic "number" is actually a location - where my shoulders are relative to the seat and bottom bracket. That deternines an arc the handlebars to be on. Now I can have those bars fairly close and low or higher and further forward and get the same back lean, shoulder location comfort, power and ease of breathing. (Again, for me, that arc is basically a straight line for the short distance I am ever going to want the handlebars on. Any closer, my knees hit. Further and my long stems go to the absurd. That line, again for me, has a "slope" of 1 cm along the steerer axis and 2 cm along the horizontal. In other words, if a 12 cm quill stem pushed low works for me, a 14 cm quill a cm higher is just as fast and comfortable. Now only one mix of stem height and length is going to give me that magic 90 degrees. I never measure or even think to note that when I look in store front windows to check back lean. I have been all over the place relative to any set arm/torso angle on bikes the fit perfectly over the past 40 years. Arms further forward and higher? They are slightly more aero. Lower and closer. Better max power for steep climbs and hard accelerations though not as good for all day out of the saddle climbs.