Remove the shock, loosen the locknut, and back the adjuster nut off all of the way to no spring tension. Then turn the nut to just take the free play out between the nut and spring. Measure that position and then turn the nut down to pre-load the spring 15/16". Tighten the lock nut, re-install, and check the free sag. It should be just about zero. When you get that, it's as good as it gets with the stock spring.
If it's not just zero, such that a fingertip will start downward motion, or maybe as much as 1/16", then take it out and turn additionally at a ratio of 2.85:1 for the desired change. Example: Desired change of 1/8". (.125"/2.85=.0439") or about 3/64" additional pre-load, measured at the spring.
Once installed, if the suspension is topped out hard under the bike's weight only, no rider aboard (very unlikely, but...) then you'll need to back it off until it just starts to move under the weight of the bike alone or if not, at least with the slightest push down on the bike.
Much more detail is available in the suspension threads. On completion it might be interesting to get static sag readings as well for an indication of what shape you're really in. I did a pretty thorough thread on this a few months ago.