is there a general rule of thumb on shocks

So my bike rides kind of rough on bigger bumps on the highway, like jarring. I thought I would give adjusting the shocks a go. I assume the number on the collar should line up the the bolt of the shock as to indicate what number you are on? Previous owner had one shock set to 4 and one to 3, I set them both to 3. I am 260 pounds and don't carry much load with me, is there a rule of thumb based on the think tank of this forum as to where I should be?

I think the rule of thumb for setting the preload is to set the sag for about 3/4". I.E. when you put your weight on the bike it should compress the shock about 3/4". More sag and you are liable to bottom out, less and it will be too harsh. Fine tune it from there. Having said that I think the stock shocks are poor.