How do I apply the interest to the equity money my ex is late paying on?

[Sindy Marie]

My ex has paid a portion of the equity money that was due in full 11 months ago. Im taking im back to court this month to ask for the remaining balance plus interest. Neither of us have attys and i realize it is to my benefit to figure the amount myself rather then ask the judge to do it. My question is how to apply the 10% interest. Do I apply the 10% the the balance or is it applied to the entire balance starting from the day it was due and bacame late? Or am i wrong on both? How do I apply the interest?