A good amount of you out there know that turning the preload adjuster does not make the spring 'stiffer' or stronger. The strength of a spring is measured by the amount of force needed to deform it by a certain amount, which is comfortable when you're working with coil springs which is what you normally have on a motorcycle. This is measured in kilos-per-millimeters or pounds-per-inch most of the time, and lbs/in is mostly used with some exceptions which use kg/mm.
Picking some numbers at random then; if you imagine a motorcycle that weighs 181kg, which is roughly 400lbs, having a set of scales placed under each wheel and held upright, the reading of the scales would add up to 181kg (400lbs)... fairly obviously. Quite what the weight distribution is going to be will depend on the sort of motorcycle is is; sports bikes tend to favor more weight on the front wheel than cruisers, but the springs in the front fork have to support that weight.
If you've ever taken a set of forks apart you'll know that the spring is slightly compressed, but not usually so much that the fork top nut is in any danger of being propelled so fast that you in any danger, but it does mean that there is a certain amount of force trying to extend the fork leg even when it is at rest. What that means is that is you have 50lbs of preload and a 100lb/in spring in a fork leg, then the first 150lbs of load placed on the fork leg will only compress it by one inch, because it has to overcome the 50lbs of preload to move the fork leg. Having lots of preload on the spring means you can have a heavy motorcycle with soft springing; lighter motorcycles that are more stiffly sprung tend to use less preload on their springs.
Taking all that into consideration , once you have an idea of how soft or hard you want the suspension to be, you can choose a spring rate (not forget to divide it by two because there is a spring in each fork leg), weigh the front end of the motorcycle (again divide it by two), and figure out the preload you need to get the ride height in the ball park you have in mind, which is known as the amount of 'sag' in the suspension.
As an aside, you need a certain amount of sag or the ride tends to be very 'crashy'. When I'm working on a motorcycle with a rigid frame, I allow one-inch of sag in the forks which seems to work fine, for motorcycles with rear suspension a bit more than 1 inch sag seems to work better, if it turns out that you actually have 1½ inches, then a ½-inch preload spacer under the spring will sort that out for you. If you're using a spacer to preload a spring you need to be sure that the spring has at least as much travel left as the suspension has. There comes a point where there isn't any space left between the coils for the spring to compress, which is a condition known as being 'coil bound' and adds a whole new meaning to the word 'crashy'. All of which seems fairly straightforward as long as you remember that the load is divided between two springs if you start playing with things.