Retract angle refers to the angular movement from fully retracted to fully extended. A 100 degree retract has 10 degrees more travel than a 90 degree retract. To achieve a particular setup, you must know the angle of the retract when extended and the angle of the retract mount relative to the ground to select the proper rectract. On many models, the retract mount rails are parallel to the wing which means the angle of the wing relative to the ground is the same as the angle of the mount relative to the ground. This is not necessarily always the case. Some models may have the rails at an angle relative to the wing itself. If this is the case, it's probably to accomodate a readily available retract. Also, in many models, the angle of the retract when down may not be perfectly perpendicular to the ground. A couple of examples would be the Spitfire and the ME109.

Scott