As mentioned at least once above (I only scanned fairly fast) the key is to spread one side at a time. Measure the existing spacing and then spread the one side by 1/2 the amount desired. Then do the other side to bring the spread to the desired spacing. Done that way there's no need to check with strings if you know the frame was straight to begin with. The issue with forcing both dropouts at the same time is there's no way to control which goes and how far. Checking for alignment following such a method would definetly require careful checking.

Sheldon's write up sort of mentions this half the amount on each side but almost casually in passing. It's not stressed strongly enough despite being A key, if not THE key, element for cold setting.