Firstly, I don't profess to be a suspension expert. There are a number of other sites and forum posts which address this better than I can. However, these are some things I've discovered over the years….
Seat hitting you in the butt can be a couple things, and isn't limited to compression settings.
First of all, make sure your sag adjustments are correct (i.e. spring pre-load). There's heaps of info on here for general recommendations for static and dynamic sag.
Assuming your sag is OK, try and figure out if the rear suspension is causing problems going over bumps or going into bumps.
To explain. Imagine a moderate, square edged, pot hole in the road. When you first ride into it, the pre-loaded rear spring will push the back wheel down into the hole to keep the tyre in contact with the ground. If you've got too little rebound dampening on, then the spring will push down too quick and the seat will pop up a bit an hit you in the butt. Too much rebound and the bike will drop into the hole a little, as the spring won't extend fast enough to push the rear wheel down. If this is the case, and the pot-hole your riding through isn't that long, you can end up having too much spring pre-load on the exit, so when the back wheel then gets pushed up out of the hole, the bike will kick and the seat will push up (assuming compression isn't too soft).
Try and figure out what it feel like dropping into and then out off a hole like that.
If the bike feels about right going off a square edged hole, then it could be that you have too much compression dampening on.
Given that your weight is maybe a little lighter than some, and if you're sag is set correctly, then you may want a little LESS rebound dampening than normal (as your pre-load won't be as high). Start with compression as recommended.
If you think the rebound is about right but you're still getting seat slap coming our of holes or over square edges (like tree roots etc), then you may have a little too much compression dampening on.
Another way to tell if you've got the rear right is how it feels when accelerating out of a corrugated corner. If the bike's squatting a bit and not getting much drive or skipping/bumping around, chances are you're spring is not getting a chance to unload properly between corrugations, which also calls for a little less rebound (normally).
There's a number of other web sites around that help to explain this better than I probably have.
Hope that's a start?
Drof