I can tell you from my own experience that with a lot of people is that a back angle relative to horizontal of less than 45 degrees, or over 70 degrees works well for limiting back pain if fit correctly with the right amount of flexibility/inflexibility, this gray zone is the middle seems to cause a lot of issues as holding this ackward middle ground angle there isn't enough support provided by the hands and not upright enough to take the weight off the core. You can kinda try it yourself by sitting with the bike on a trainer with your hands off and moving the back through the range of angles