Since I use the same saddle on all my bikes, it's pretty easy to get the reach and drop right.

I set my saddle angle (about 1/2 bubble nose up), setback and height relative to the pedals first.

I then setup the brake levers on the bars, and the angle of the drops as identically as I can (different bars, different levers, but I can get it close).

I then pick my stem length and angle (and/or spacers) to duplicate the reach and drop to the bar that I like. Since the saddles are identical, and they are set up over the pedals exactly the same, I can just use the nose of the saddle for the reach and drop measurements.

If a person uses different saddles on different bikes, it might be a little trickier. But once I found a saddle I liked I can't think of any reason to have different ones on different bikes.