Formula / model

We are looking at ways to pay employees a bonus based on service, we have the maximum amount we want to pay out. What would be the best model to work out what we should pay per person? 

I have the staffs current annual salary, the days, months and years they have worked for the business. 

I need a model that can be played with, currently we are looking at depending on the category you fall in to you will get a % of you current annual salary paid but this % might change so I need a formula / model that has room for these changes.