The bell curve grading system is a flawed systems.

My company is using it.

Eg, top 5% get A, next 10% get B, last 5% get D, the rest all get C.

D = 0 bonus
C = 3.x bonus
B = 4.x bonus
A = 5.x bonus

of course if you get A, your up coming annual increment will be the highest at 5+%
B = 4+%
C = 3+%
D = 0%

same for promotion, A grader will get a higher promotional increment % than B, B>C ...etc


For a fairer system, bonus should be pay out equally throughout the board....ie everyone get 3 months of bonus regardless of gradings, those with better grading to be promoted faster or give them more annual increment.