Only point I'll add is that no ruler or caliper is ever needed when spreading a rear triangle. All that's needed to insure a centered result is the intended wheel (which serves as the drop out width measurement device AND no more then that) and any thing that is straight and stiff enough to act as a gage when held in one's hand to compare the two sides of the string test (I use a pen often for this) .

As you bend the rear stays/drop out apart bit by bit you both test the width with the wheel and the centering with the classic string test. Neither needs any rule with MMs or inches. The wheel's axle is a go/no go gage. The straight stiff device held in hand, one end touching the ST and marked where the string pass by with your fingers/thumb is simple brought to the other side to compare.

Of course using some measuring devices makes you feel like you're being "accurate and precise". And when i do this in front of paying customers (which i don't like to as most people don't have the understanding of bending frames and the lack of concerns if done well) I use my Mitutoyo calipers. Because it makes me look all professional. But really a go/no go gage and an comparative indicator are just as good. Maybe better in that you then work to a single result and not get caught up in numbers that have no real meaning. Andy.