How would you guys calculate the killing efficiency of a unit?

Would you use (No. of kills unit made)/(No. of killed in the unit)?

I would like it to be a percentage instead of a ratio, and would like it to be dependant on the acitivity of the unit, so that a unit with 30 kills and 15 dead would have a different eff. than one that killed 2 and lost 1. Do you think I am off track here?

Thanks for your help...