How to handle bonus proration for multiple assignments

Summary: We have a requirement where we have multiple assignments and have bonus proration. We are planning to use fast formulas to show past 5-6 jobs along with proration factors for each job. But in case of an employee having multiple assignments, there will be multiple rows in the plan worksheet and due to the fast formula, each row will show the past 6 jobs and calculate bonus multiple times for the past jobs in FF. Also then how will FF deal with past multiple assignments? Any faced with a similar issue? Any alternative solution or approach will be of a lot