Read my last paragraph again. I am not trying to calculate reach or I would simply set the front of the saddle to be 53cm from center of bar and be done.

I could set it to be 56.2 (53+2+1.2) and hope that gets me the same saddle setback, but I don't know if the difference in STA, stem angle, and seat-to-bar drop make a difference. Thus my question.

As for getting the same setback as on my last bike, I don't have a reliable way to get the bike level and measure that, so this is a workaround.