It shouldn't be this hard. I just out on a new spring on my rear shock. Somebody said "don't forget to set the sag". What the.... So I watched videos and read articles about sag. What am I measuring? One guy at a reputable store had a good video. Just pick a point to measure it to. Well if I need 93-100 of sag I can just find somewhere to measure that to. Bikes are different. What am I missing? I know how to hit the circle thing with a hammer and screwdriver. It's back on my bike.
From where to where and what do I measure?

 

You measure from the axle straight up to a point along the arc of the travel of the axle in that arc. However, let's make it easy. You really can't see that imaginary line of the arc of your axle, so just measure straight up, perpendicular to the ground...with your yardstick, tape, etc. leaned forward just a tiny bit. The difference between the exact location on that arc and what I'm describing is insignificant.

Now, you know you're measuring from the axle, a very specific point. So what location on the bike do you use? You can use any point along that line of your yardstick or tape that you can clearly identify. It can be a bolt in a side cover, an edge of the fender, or whatever. It's not important as long as you can continue to identify that spot during this process. If nothing else put a Sharpie dot at some location.

I think this is what you're asking about...but maybe not. I won't go into the process of how you measure a static bike with no rider and a loaded bike with the rider. I'm assuming you were clear on that.

 

On my last bike I just used the edge of my rear fender just behind my seat. Its REALLY not that critical, just get it close, ride it and adjust if too soft or stiff.