Image "The Badger Bernie Sulzburger"
42deg back angle 87deg reach.
There seems to be a lot of opinion about the reach and stack height of the handlebars, with a a back angle range of 37-50 deg and Arm reach of 80-93deg.
Any quick U-tube search on this subject offer such a variety of information that it is impossible for riders to understand the reasoning behind the various approaches, one U-Tube video would have you believe if you sit upright with your arms out at 90deg to your shoulders and simply fall forward this is the correct position on the bike. The person also claims to be part of British coaching unfortunately British coaching hasn't heard of him and he is NO record of ever having completed any coaching credentials in the UK.
So why the range of angles, first we need to accept that muscles need to lengthen in order to contact and produce the energy required to propel the pedal rotation, presuming the saddle is in the correct position a rider with a 50 deg back angle has little extension on his posterior muscle chain and a rider with a 37deg back angle simply has to much tension. ProBikeFit uses a range of 42-46deg depending on the riders biomechanics and their goals on the bike. A rider with a 42deg back angle with have a great arm reach angle 87-88deg reach than a rider with a 46deg back angle who with have a smaller arm angle of 80-82deg, in both set ups the riders reach ultimately will stay the similar and only the saddle to handlebar drop will change.
Riders who are Mountain specialist and TT specialist such as Richie Porte and Chris Froome are the only 2 rider I currently know of who use a 40deg back angle, while riders such as ex World Champion Julian Alaphillipe ride with a 44deg back angle.
Due to the constraint of modern living it is my onion that most riders are better of with a 44-46deg back angle due to the limited range of motion most riders display when assessed.