Hi, I have solved this mystery finally. I had tried to get an answer from North Plains on this subject as my siblings and I were owed interest on a well in McKenzie County, North Dakota. We tried to get this answered for 2 weeks. FINALLY today I was told this is how to figure your interest owed. You take the amount of check that was late in payment times the amount of days late, divide that figure by 365 days (days in a year), then multiply that amount by 0.18%. There is the interest amount owed to you. 18% is the amount of interest owed when payment is late in North Dakota. Hope this helps.