How about this.
Each post is worth 1
1 cheer makes it worth 1.5
2 cheers makes it worth 2
3 cheers makes it worth 4.
4 cheers 8
5 cheers 16
1 jeer makes it worth 0.8
2 cheers makes it 0.4
3 cheers makes it .2
4 cheers make it worth 0.1
Cheers and jeers cancel each other out.
A score of 8 or more required for POTM nomination. Whichever nomination gets the most net cheers is POTM.
This gives you an ever increasing number, if that's your thing.
You could divide it by Total Posts if you prefer.
This system could work with just Cheers too.
To add an interesting wrinkle, you could limit the number of ratings that can be cast per day based on your total score (so more experienced, or well regarded posters get more power).