The best way to measure sag is with 2 people. BUT if there are not two...here is how I did it.
I taped laser measurer to fender bracket. Taped some aluminum tape to the inside shock where the bolt goes through. I lifted the rear, wrote down that measurement. THEN, i put my cell phone camera on a stool, between to blocks of wood to steady it, and set it to record video. From there, I lowered my bike, sat in the seat for several seconds, then, got up, turned off record on phone and played back to see the reading. Each time I did this, I made sure the laser did not move, by checking the measurement with the bike jacked.
I taped laser measurer to fender bracket. Taped some aluminum tape to the inside shock where the bolt goes through. I lifted the rear, wrote down that measurement. THEN, i put my cell phone camera on a stool, between to blocks of wood to steady it, and set it to record video. From there, I lowered my bike, sat in the seat for several seconds, then, got up, turned off record on phone and played back to see the reading. Each time I did this, I made sure the laser did not move, by checking the measurement with the bike jacked.