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i think it you start from standard and then measure the sag you will be able to determine whether you need to turn the preload up or down.

Quote:

Rear Preload

Whatever your weight, when you sit on the bike, you want the bike to squat just a little bit (about 30mm) but not too much. You want to ensure that the bulk of your rear suspension travel is available for when you actually ride the bike as opposed to supporting your heavy frame. At the same time you should have a little bit of sag left. If the bike drops too much then increase preload and if you are very light and the bike barely moves under your weight, then soften the preload.
Below is a more detailed explanation. This is easy to set up but can be a bit confusing so just follow the steps one at a time.
http://www.moto-racing.co.uk/Graphics/sag1.gifhttp://www.moto-racing.co.uk/Graphics/sag2.gifhttp://www.moto-racing.co.uk/Graphics/sag3.gifStep 1Step 2Step 3
Step 1: First find the fully UNLOADED length of your rear suspension. Put your bike on its centre stand. If you do not have a centre stand find a means of lifting the rear wheel up under the engine so that there is no weight (including the weight of the bike) on the rear axle and wheel. Measure the distance between the rear axle and a fixed point directly above like a bolt or mark. Note this measure measurement. Do not use a rear wheel bike stand as there is still the weight of the bike on the swing arm.

Step 2: Find the NORMAL length of you rear suspension. This is amount the suspension drops under the weigh of the bike alone without the rider. Put the bike on level ground and bounce it up and down to free any stickiness. Measure between the same two points as above, i.e. the rear axle and the fixed point directly above.
Step 3: Find the fully LOADED length of your rear suspension. This is the length of the suspension with the rear wheel on level ground and the rider seated on it in his normal riding position in full biking garb. First bounce up and down on the seat to loosen the suspension then get into your normal riding position with all weight on the bike and both feet on the bike. To prevent yourself from falling of (if you have not already) lean the bike against a wall and get your mate to measure between the same two points as above i.e. rear axle and the fixed point directly above.

Next thing is to understand a couple of buzz words. “Static sag” and “Rider sag”.
RIDER SAG is the difference steps 1 and 3. This is the amount the bike drops when your heavy **** sits on the bike.
STATIC SAG is the difference between step 1 and step 2. it is how much the bikes weight acts on the rear suspension or how much you can lift the bike rear without the rider on it before it tops out.

A top racing suspension company suggest that rear wheel rider sag should be around 30-40mm and static sag should be around 5-10mm. However this many vary depending on your bike and manufacturer. The more rider sag you have, the softer the bike suspension will be. I personally use 35mm for road and a few track days use.
Therefore if your rider sag is less than 30 mm then your preload is too hard, if it is more than 40 mm, then it is a bit soft. Once you have set your rider sag as close as possible to 30-40 mm, next check your static sag. If you have more than 10mm then you may need stiffer springs. If however, if your static sag is less than 5mm or you have no static sag then your springs may be too hard for your weight.

If you have to compromise then try to have at least a bit of Static sag in order to stop the bike from topping out.

Front Preload

Some people set the front suspension based on rider sag and static sag. This is especially useful in determining if you need softer or stiffer springs. Use the same system as above to determine Rider sag (35-48mm) and Static sag (25-30mm). However I prefer the below method to optimise my current original front forks.
You want your bike to use as much front suspension "travel" as possible without "bottoming out" (no more travel) even in extreme conditions. So lets establish how much suspension travel you are currently using.
http://www.moto-racing.co.uk/Graphic...-cable-tie.jpgWrap cable ties around the smooth part (stanchion) of front forks near the rubber seal to the forks. Ensure that it does not scratch the stanchion and it is not too tight as to damage the rubber seals.
With the cable ties in place ride your bike as normal using as many riding conditions (corners, braking, accelerating, wheelies and stopples) that you are likely to meet. Then increase or decrease your preload until the cable tie stops about 10 mm before “bottoming out”.
Reducing preload should give you more travel. If the forks bottoms out, reset the cable ties and increase preload until the cable tie stops around 10mm above the limit. You should always leave about 10mm travel for emergencies like slamming into uncharted potholes.
Done