It is important to set sag specifically for the frame, and not just assume that 30% shock sag = 30% suspension sag, because that is very rarely true.
Below you will find simple graphs that outline how shock compression equates to suspension sag within recommended range.
Remember to be constant when setting the sag. Next to a wall for balance get into riding position fully kitted up (pads, backpack with contents, helmet etc etc) and carefully make sure not to overcompress the suspension from this position as you get off. Then once off the bike, measure the gap between the o-ring and the shock seal to get shock compression at sag point. There is no such thing as an optimal sag for everyone, it depends on the individuals riding style and the terrain and speed that they ride at, so have fun experimenting and find what works best for you!
