I found my old Performance Bikes Suspension Set up guide, so I thought I'd post some of its wisdom.
Sorting the basics
Before starting to change settings, check for other issues that affect handling…
Worn, mismatched, misaligned or over/under inflated tyres
Knackered headset, swing arm or wheel bearings
Blown fork or shock oil seals
Bent frame, swing arm, forks or wheels
Once all this has been checked, it’s time to set the static sag. This is the ride hight of the bike and is adjusted with the preload adjusters. This process will need…
A friend
A ruler
A long stick
Some cable ties
And possibly stands
Starting at the front, tie a cable tie around fork leg, lift the front off the floor and push the cable tie to the fork seal. Gently put the front down, sit on the bike and lift your feet and get in a normal riding position (with a friend holding the bike up). Measure the gap between the tie and the fork seal.
At the back, mark a point on the sub-frame or seat unit directly above the rear wheel spindle with the wheel off the floor. Run the rule from the marked point to spindle, sit on the bike in riding position with the wheel back on the floor, have a friend measure the gap again.
You now have your static sag figures. They should be 30% of the suspension travel, normally 35-45mm. Adjust the preload and measure again until your sag is within this range.