I'd think the important part of measuring is to know that this is just to get in in the ball park and that you will likely need to adjust it multiple times afterward.

Me? I'd measure from where I normally have my sit bones on the saddle and try to imagine where that might be on the new saddle.

If this is to transpose measurements a fitter may have given you, then I'd ask the fitter where his or her measurement is to be taken from.