Partial Payments on Open-and Closed-End Credit
Institutions should use one of two methods to recognize partial
payments. A payment equivalent to 90 percent or more of the contractual payment may be considered a full payment in computing past due status.
Alternatively, the institution may aggregate payments and give credit for any partial payment received. For example, if a regular installment payment is $300 and the borrower makes payments of only $150 per month for a six-month period, the loan would be $900 ($150 shortage times six payments), or three full months past due. An institution may use either or both methods in its portfolio, but may not use both methods simultaneously with a single loan.
Uniform Retail Credit Classification and Account Management
Policy
http://www.occ.gov/ftp/regs/2000-20a.txt