Basically I shouldn't be setting preload based upon sag settings for the front end if there is a long top out spring
. Instead I should trust the chosen spring rate to be correct for my riding, and adjust preload according to how much travel is being used for my riding style (i.e. zip tie on the stanchion). This should be in the ballpark of 8-18mm installed spring preload.
It gets complicated and Dave Moss etc go into this in his resources. Long top out springs can increase the sag readings, and if you try and crank it out with preload you're basically fighting against this spring and potentially creating harshness / traction loss on acceleration. The stock cartridge runs a short top-out spring (see @stroh forum post for pics) so you can probably use the traditional 30% travel method there as starting point.
I'm happy to stand corrected by more knowledgeable people like Matt etc.