I am fairly certain that you are incorrect about "Reach". Rather it is defined as being the horizontal measurement from a point plumb above the center of the bottom bracket to the center of the head tube on level. Then setback is defined from the same point above the bottom bracket rearwards on horizontal to the center of the seat tube. That way, effective top tube length = reach + setback. Setback is especially helpful in deciding if the frame will let you move the saddle rearwards enough to get your knee in correct position over the pedal axle, while reach and/or effective top tube length are helpful in deciding if the cockpit is long enough with the right stem length for your upper torso.