Do you have a formula/method you use to calculate Christmas bonuses for your staff?

We have implemented a performance based Christmas bonus.  We reward based on if they met their goals. The goals for our team members is 95% or above customer rating, above 85% production, complaints from clients.  These are weighed at different percentages and if they don't meet the goal then they don't receive that portion of their bonus.  It seems to take out the "I like you or not factor".