You may want to start by checking your preload system by turning the adjuster full out or counterclockwise. Then start turning it clockwise noting how many clicks till you feel resistance when turning the adjuster. A fully filled preload system will have less than 4 clicks which is 2 turns. You may find that you have many turns till resistance. If so you can fill the preload adjuster with oil (hydraulic jack oil works) There are a couple of how to do it on the forum. By having a filled system you will then get full use out of your spring preload. Then as suggested above start in the middle and go from there.
I am not sure why one is not suppose to fully use the complete range of the preload. The shock designer would not have given it full range if it was not suppose to help with a heavier load. Not that it matters, but I use to run at full preload, till I filled my system. Then around 60% of preload, I ride single.