Formula for interest rates

Hi guys, I posted this a while ago and have asked everyone I know and I still can't work it out!! Is it even possible?

I am trying to work out the interest rate needed on a bank account in order to pay off a loan in the future.

I want to know the minimum interest rate to pay off the loan at a date which is variable without going into the overdraft when the loan is paid.

The attached spreadsheet is a simplified document.

Many thanks in advance,

Alex

Please bear in mind that the cashflow will be changing so therefore you cannot just work from the starting balance, it needs to take the bank balance into account each month

Here is what I came up with without taking into account the bank balance on a monthly basis.