[InvisiBill]

You can actually calculate how much to cut off the spring to give a desired rate.

new spring length * new spring rate = original spring length * original spring rate

If the stock spring is 450mm long (that's just an example - don't actually use that number) and 0.65kg/mm, that gives you a total value of 292.5 for the stock spring. If you want it to be 0.75kg/mm, you just divide by that and get a new length of 390mm. For 0.85kg/mm, you'd end up with 344.1mm. If you want to get really precise and go for 0.7945kg/mm, cut it down to 368.16mm.

The only downsides to cutting springs are that it's a permanent mod, and if you're really heavy, you may not be able to shorten it enough without interfering with operation of the spring (obviously you can only cut off so much length before it doesn't work in the fork).

You'll need spacers to make up for the removed length of spring. The "extra" length of spacer in the fork tube (in either stock or modified form) is what creates preload. That's probably not the correct setting from the factory, and it will necessarily change when you change the spring rate. You'll probably have to tweak the length of your spacer to get the correct preload/sag setting. ProTip: It's easier to trim off a little than it is to add a little back - start longer and shorten it.

If you have the preload properly set on the stock spring, you can compare the total length of the spring & spacer to the total length of the fork's spring chamber to figure out how much force it's applying on the spring. You can then use that to adjust your spacer length to give the same force on the new spring's rate.