Late payment penalties (Calculation)

Friends
plz find attachment for my problem. It has two parts LPC(Arrears) and LPC(Fixed), i have developed column for LPC(Arrears) but not able to proceed furhter for LPC(Fixed) which is dependent of LPC(Arrears) calculations.

Attachment consists of monthly bill( with payment due date 14th or 15th of succeeding month) and payment details of customer in parts.
LPC on arrears is applicable to the balance amount on arrears and whole month whereas LPC on fixed cost is applicable only after 14th (or 15th) and after full arrears is been paid.

Plz see this calculation sheet and help me out.
Calling Xcel gurus ....