The bonus should be based on a few fundamentals......Attitude for a start, desire to look after company assets (treat them better than if you owned them yourself), desire to look after customers, Attitude, Attitude.......be continually safety concious and not sweep things under the carpet. Ohhh and attitude!
Employees with a good attitude may not be the best technically speking at first, but they can be trained and gain skills etc. A smart ar$e with a bad attitude can never learn anything.
.........now how do you nail a measurement to $$ I do not know. maybe a score for the number of times they p!$$ you off each year, each time its worth 2% points. If they average one a week there bonus is gone!
J