D. Forfeiture of the interest of a purchaser in the property for failure to pay monies due under the contract may be enforced only after expiration of the following periods after the date such monies were due:

1. 1. If there has been paid less than twenty per cent of the purchase price, thirty days.
2. 2. If there has been paid twenty per cent, or more, but less than thirty per cent of the purchase price, sixty days.
3. 3. If there has been paid thirty per cent, or more, but less than fifty per cent of the purchase price, one hundred and twenty days.
4. 4. If there has been paid fifty per cent, or more, of the purchase price, nine months.

E. For the purpose of computing the percentage of the purchase price paid under subsection D of this section, the total of only the following constitutes payments on the purchase price:

1. 1. Down payments paid to the seller.
2. 2. Principal payments paid to the seller on the contract.
3. 3. Principal payments paid to other persons who hold liens or encumbrances on the property, the principal portion of which constitutes a portion of the purchase price, as stated under the contract.