Sirius, I don't think bow length is TOO important unless your style uses the string as a reference like touching the eyebrow, glasses, nose, etc.
As far as the 90* angle, just mentally look at the bow arm and the spine (you kind of have to imagine where it is) of the shooter. Try to set that at 90, then if shooting at a lower target, move the spine while maintaining the bow arm at 90 to the spine so your sight picture is on target. Is that what you are asking?
Arne