I just swapped out my Terry Butterfly for a Selle San Marco Atola.

Does anyone have any tips for how to know of the saddle is too far forward or back? I tried to line up the new saddle where the old one was, but the shape and size were so different, I really have no idea if I got it right. And it doesn't look like the markings on the rails are intended to be consistent across saddles by different manufacturers. My old saddle was set pretty far back, so that's where I put this one.

I know there is something about a plumb line from the knee over the shoe on the pedal...