".. it is 100mm from when the bike is up on the stand (no weight on the wheels) to me sitting on the bike; is 100mm of sag"
Make SURE your initial measurement is with NO LOAD on the suspension. It has to be 'topped out' to make the initial measurement. It doesn't have to be on a stand to do that. That's a bit of a problem measurement-wise 'cuz you can't be measuring to the ground if the wheel is at all OFF the ground.
Settle for lifting the rear fender til you feel it top out...THEN measure to some other fixed ref point...like the axle. Measure at 90º to the fixed point! Helps to make a bit of a mark on the fender where you measure to. You'll need to be measuring to the same point when you're sitting on it.
Obviously, this is done correctly with a couple of people. Hard to measure when you're sitting on it. You can use a couple of bike straps to hold the bike upright so you can sit on it with no toes down. Sit up in the saddle where you should be. The difference between the two measurements is your race sag.
Using the same measurement process, check how much the bike drops with its OWN weight. That's FREE sag. Should be between 1/2 to 1". If it's less than 1/2", you have too light a shock spring. If it's MORE than 1", you have too stiff a shock spring.
You must set race sag FIRST..then check free sag.
At 180#, the oem 5.0kg/mm shock spring should be just fine.
"Don't put off the sag adjustment"
Amen!! It's free. It's imperative. It can change the handling of your bike every bit as much as a $700 suspension job. Kinda like greasing your suspension.....you just GOTTA do it!!!
imo..............;)