What is the interest rate in your last example. Why have you added a new example with no explanation. How does this compare to the examples in your post 1. Incomplete information results in no answer.
I don't understand your interest calculation. Normal calculation would be Outstanding Balance times interest divided by 12 as you are charging monthly. I believe that because you are using some other calculation, using Excel formulas to calculate Present Value arrives at a different value. I am stumped and will leave this for someone else to help.