Sag is probably the most fundamental and important aspect of setting up your suspension. In case you didn't know, sag is the amount the suspension compresses under the rider's static weight, usually expressed as a percentage of the total travel. The brilliant thing about sag is that it's transferable between riders of any weight and bikes with any amount of travel. Running 30% sag will give a roughly comparable ride feel for a 120kg rider as it would for a 60kg rider, even though the heavier rider will need roughly double the spring rate (spring stiffness or air pressure) to achieve the same sag. Similarly, if you have twice as much travel you'll generally want to run about twice as much sag (half the spring rate), which means the same percentage
But when it comes to dialling in your particular bike, it's worth being precise with sag measurements because a small change can have a big effect. If you go from 30% to 27% sag, that may sound like a pretty trivial difference but the amount of sag has actually changed not by 3%, but by 10% (3/30). So, the spring is 10% stiffer with 27% sag as it is with 30%; that's a very noticeable change.
There are two ways to measure sag - either seated or in the attack position. Measuring sag in the attack position (standing on the pedals with elbows bent) is more representative of how you actually ride, but it can be hard to measure sag like this on your own without pushing the O-ring past the sag position as you get on your bike. Measuring sag seated is much easier - just slowly sit your weight onto the saddle, lift your feet in the air briefly, then without bouncing, put your feet back on the ground and dismount, then check the O-ring position. If measuring sag seated, you'll typically measure a little more sag than when standing, but so long as you measure it the same way every time, it's a good reference point.
Another caveat is that with a progressive suspension linkage, 30% sag on the shock correlates to slightly more than 30% of the wheel travel. This is because, with a progressive linkage, the wheel moves further for every millimetre of shock stroke early in the travel than it does later in the travel. It's not worth getting too hung up about this though because, for a moderately progressive bike, the discrepancy between shock sag and wheel sag is only around 2% (30% shock sag correlates to about 32% wheel sag). Also, if you have a more progressive linkage, you might want
to run a bit more wheel sag than with a linear one, so setting them up with the same shock sag isn't a terrible starting point.
Something which is more significant but more rarely discussed is friction. When you apply your weight to the bike, the suspension compresses until the force from the spring plus the friction in the shock and linkage is enough to hold up your weight. So if there's more friction, you'll measure a smaller amount of sag with the same spring rate. You can test this by measuring sag in the normal way, then have someone push down on you to compress the suspension beyond the sag point, then have them slowly release that downward force so the suspension extends until the downward force is removed. You'll find that you measure more sag when the shock extends than when it compresses - sometimes a lot more. The difference is due to friction, so this test can be a useful way to compare friction between bikes or identify sticky shock bushes or seals. Friction is even more significant in the fork, which makes measuring fork sag almost useless in my view.
Traditionally, it was often said that you should run about 25% sag for cross-country and about 30% for everything else. But modern bikes generally have more progressive linkages and radically better air shocks with volume spacers to control bottom-out. On the other hand, people are riding shorter-travel bikes much harder than they used to. So, are these basic guidelines still in the right ballpark, or are they way off?