Originally Posted by
weeniecyclist
I would post the photo here, but need a few more posts before being able to post links and images.
I think it's a bit more than 90°, when drawing an rectangle laying it over my back and the upper arms.
Then your guess about reach is correct. You could try moving your hands back in that same position and seeing how far you had to move them back to get 90°. which would tell you how much shorter a different stem would need to be.
It's weird that the 54 will fit almost the same except for more drop. Is that desirable for you? If you get on the bike in that horizontal forearms position and pedal, is there a decent gap between your legs and torso? Would you like to be lower or not?