You need to stick to your contract. If you put anything in the claim that is not in keeping with the contract, you lose credibility. Small claims decisions are based on balance of probabilities and not 100% fact as opposed to criminal court cases. Per day translated into per annum is 365%. And I assume you are charging simple as opposed to compound. So....$10.00 @ 365%/annum, simple, for default of 45days would be: $10.00x365% per annum/365 days*45 days due = $4.50 interest.