Planning and Forecasting Tool

Dear Community - I have been asked to develop a planning and forecasting tool to plan and monitor funding throughout the year.

Here is my issue ....... if the course is a 10 month course then the 10th payment is a percentage of the total and i am not sure how to do this. ie, how to make the 10th payment different from the rest.

If this is vague then i can elaborate further. Thnaks for looking.
D