BTW... The reason your European friend was so fast:
You only have to make ONE adjustment (although, until you develop some confidence in the method, you SHOULD double check both diagonals. As I ALWAYS warn, your calculations could be off. Checking measurements is always cheaper than writing another check for more material.)
You simply divide the difference between your two diagonal measurements by 2, and move one corner of the frame to that measurement. It's done. The other corner will automatically be the same.
"True eloquence consists in saying all that is necessary, and only that which is."
François Duc de La Rochefoucauld