I use this to help measure from the seat to the handlebar.

I go by knees and elbows. With my hands in drops or on hoods, your preference, and forearms horizontal, I level my pedals and see the relationship between knees and elbows. Hopefully I have one bike on which I like the reach, so then I make that relationship the same on other bikes.

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.

I run a string or thin electrical cable across the saddle. Tack it down with a length of tape. Move it back & forth until it feels like it’s centered under the sit bones.
That gives me a reference to get any pedal lengthwise correct WRT the BB, and the bars right to the saddle.

Very good ideas. I must try them, because I have a couple of bikes that need adjustment, and I don't know which way to go.