Range doesn't have to be 5 and -5 thats just the values I used in HandSaw mod, which was the first mod to use range, so every one just copied me =p
as for:
The rotation can be any number, it will automatically work it out, e.g. 400 -> 40
Negative masses have a tendancy to go through the floor and take you with them
If you grab a negative mass it will seem a lot heavier than, e.g. you will move around it
I never mentioned negative masses because it's not somthing thats supposed to work. So negative masees would fall into the "known bugs" section, but I've already covered it in the "don't make somthing to light" part.