Put the spring in the collapsed fork.
Screw the cap completely onto the rod.
Extend the rod.
Measure the distance from the top of the spring to the bottom of the cap.
Any spacer length longer than the measured distance will be your preload amount.
Yes, I did leave out the washers for simplicity. Measure them (around 3mm), figure them into the matter. Which is to say subtract their thickness from the measured distance.
Cut a piece of 1"pvc pipe to fit. Cut it square.
You can put in oil as a measured volume. This does not take into account how much oil you didn't get out in the first place.
Oil level works better as a measurement. Make it 110mm from the top of the collapsed fork (spring out).
110mm = 4.33", or about 3 5/16"
Take a look at CDave's site (listed at the 'every kdx rider should read...' thread at the top of this forum) for more info on fork oil level and how to measure it.
Try a search.