The Pactix interest clause sets the standard interest rate at 5% per annum simple interest. That's low for some commercial transactions, but it has the advantage of being reasonably safe under the usury laws of nearly all U.S. jurisdictions.

Heads-up: Be sure to check with counsel before specifying a higher interest rate — in some circumstances, violation of a usury law can have consequences such as cancellation of the underlying debt, and intentional violation may have criminal-law implications.

The Pactix clause provides that interest begins to accrue at 30 days past due, because some jurisdictions (for example, Texas) restrict charging interest before them.

For additional information about state commercial usury laws in the U.S., see Appendix A of a December 2004 report by the Texas Senate Business and Commerce Committee (scroll down to page 21). The 'Lectric Law Library has an undated report summarizing the permissible interest rates under state law.