Calculate royalty percentage per transaction based on a cap

[Guest]

I need to determine the Royalty Percentage per transaction in order to calculate the Royalty Amount per transaction. Example: Royalty Amount = (Gross Commission Amount x Royalty %)

The issue is that I have a Monthly cap for the Total Royalty Amount of $1.000.00 (per month). I can have one or more transactions in a month with various amounts of Gross Commission.

[Guest]

Which comes first, the chicken OR the egg