Noami,

You have numerouw way how to calculate the interest, not only 360 / 365 / 366 days , also including the day of start day of end a.s.o. In my opinion before you decide to a final formula, better present your customer with your idea anyway the might have there own business ruling. In any case be sure to make the calculation itself to an isolated procedure so that you can switch afterwards.

Good luck.

Regards,

Koen