There really isn't just a 'right' answer. It all depends.
Easy way to see if you're close enough to start is to;
1. Check the free sag, that is, with the bike about vertical, with just the weight of the bike, you should be able to pick up on the back of the bike about 10mm before it tops out. Plus or minus a couple mm is fine.
2. When you get on the bike, the rear should go down about another 20mm or so. This would give a total sag of about 30mm. Again, plus or minus a couple mm is fine.
Measure this along a vertical line directly up from the rear axle to someplace on the tail. You cannot do this on a rear stand- the added friction screws everything up.
If that adds up about right, it's close enough to start.
DO NOT SACRIFICE FREE SAG TO GET A TOTAL SAG NUMBER. YOU NEED TO HAVE ABOUT 10MM OF FREE SAG. If the total sag is wrong, change the spring. Know this, sag is just an observation of spring rate and preload- it is NOT a final answer.
