Another error found today, luckily before I paid a prepayment.
I realized that I had been lazy in my estimations. I have been using a yearly lump sum prepayment as default setting in my amortization tables. When I used the quarterly amount, even with our interest rates quite low, it still makes a good amount of difference. I found out I was over paying again!
So I made some corrections to my spreadsheet and freed up some more cash flow. In reality, we are paying prepayments bi-monthly so the final effect will still be an overpayment but the calculator I am using doesn't have anything less than quarterly.
I have been feeling nervousness about cutting things too close so I am grateful my last few errors have meant less financial tension.