Setback is measured from the saddle tip traditionally,but I wonder why.
I have recently switched to a shorter nose saddle---given that I spend most time on the back 1/3 of the saddle,it seems to me logical to measure from there,in order to get the same relationship to the crank.
In fact some saddles have no nose at all---not that I would use one,because you do sometimes need to slide forward.

Because it's easy to do. If you want to make a small adjustment to the fore/aft position, you can measure to the center of the bars before and after the change.

If this measurement is to have any value, you need to use the same saddle if you have multiple bikes. Between different saddle models the measurement is only ball-park. I tried 4 different saddles last year and they were all within 1cm of the same, once setup to place me in the same position, but none were the same as my old saddle.

If you want real accuracy, you have to perform a KOP measurement with each saddle and even that is not easy to do with a great deal of accuracy. A person won't get on a saddle and sit at exactly the same location each time. Riding the bike on a trainer with the bike set level is probably the most accurate.

All my saddles are different length. I bailed the nose measurements. I now use the center of the main sit area where my sit bones go. I just use the logo if there is one or a small mark under the saddle. An even better way would be to get a line through the seat tube with a laser and measure where it intersects the seat. GL

My thoughts exactly---perhaps the rear 1/3 front 2/3 junction is a good start point.

You might want to check your KOP position. Maybe a seatpost with lots of setback .

I measure from the center of the bars, to the BACK of the saddle edge. That's where I wind up sitting, and it doesn't get messed up by longer or shorter nosed saddles.

It's not better than measuring from the front, if the saddles are different models. They all have different shapes and widths the can place the rider in a different position. A check of the knee to pedal relationship is the most reliable way to verify your position.

I follow my own advice and use the same saddle on all of my bikes.

I just ride a lot and make incremental adjustments over 500 - 600 miles (1 month for me). I start with a baseline that works for me. I have a good sense of body position and can feel very small changes pretty easily. I know what's comfortable, and dial it in by over and undershooting movements until it's just right. It's iterative because i'm working height, set back, and tilt. A change in one may necessitate a change in another. When i can't improve it anymore, it';s done. This is starting from a perfectly acceptable baseline position, which I do not measure. I'll usually keep a change for at least 50 miles (2 days or so), unless it's obviously bad.