The setting is really subjective. There are formulas for sag, but those are generally for competition or those that ride hard. People will tell you that it has to be set a certain way, but it is all based on how you ride, the roads, and load. Not what someone else uses for the settings. Ride easy on rough roads and you want it softer than riding hard on smooth roads. You need it a little stiffer with a load. I would keep it a little on the stiff side with your weight. Too much sag and the bike won't handle well, but unless you are riding hard you won't notice it.
You shouldn't slide ahead in the seat if the suspension is too soft. It is probably because of your passenger pushing you ahead. Too soft and the rear should be lower so you would have to be sliding up hill.
Just find out what seems to work well for your riding style. Too much sag and the front end will want to go straight in corners. Plus you will bottom out on big bumps. Too stiff is better than too soft, but the ride will not be as smooth, especially on rough roads.
Write down your settings so you will know what changes do for you. After a while you will figure out where to make the settings for the condition. Or you can just set it for what someone on the internet tells you.