hi there, check out the justkdx site as there was an article on there about making spring spacers. but in a nutshell the main points with spacers is that you have to calculate the length, and the spacer length is not the same as the preload length (well sometimes it might be but thats just chance..)
to get the spacer length you need to subtract the spring relaxed length from the fork internal length when assembled (this is the available length that the fork spring can fit into the fork when the cap is on the damper rod but cap is not screwed onto the fork body) sometimes with xr or aftermarket springs this will be a negative figure.
then you add the preload amount you want to use say 10mm (to get this figure you are suposed to work this out from the amount of fork sag when everything is put back together, but a lot of people just try preload in the 5-15mm range...)
so finally by adding the difference between the fork length and the gap you want them to fit into, plus your preload amount you get the spacer length.
in my case with xr springs my spacers ended up being 10mm long from memory.
hope this helps,its a hard thing to explain without pictures or a diagram but if you do a search on here you will find more info or some links.