I cannot get a formula to work in excel. It has a lot of math equations to be solved in sequence through parenthesis. I can make it go only so far. Basically, I put the last parenthetical on there and then I get a pound sign result. Here is the math I need; how can I make it work?...

=((E6/1200)+((E6/1200)/(((1+(E6/1200))^E7)-1)))*E5

E5 = $66523.25
E6 = 6.8%
E7= 120 (payments)

This is the math itself… (.0056666666+(.0056666666/((1.0056666666^120)-1)))*66,523.25 and that should = $ 765.55175


The REAL Problem:

This worksheet is obviously for a 10 year loan with 120 payments. Right now I am just using the pre-made amortization formula with the combination of =PPMT and = IPMT. However, this series of formulas merely kicks out a pre-made number for each of the 120 payment installments based on the initial life of the loan and the planned payment schedule without reflecting any changes made during the life of the loan. If I make an additional payment, that amount will not be reflected in the interest due on the new principal amount remaining, or on the amount of payments left as I chip away at the principal ahead of schedule. I want each cell to draw from the total amount paid, and the principal amount remaining, so that I can see the life of my loan. I want a column for my monthly payment, which would stay constant based on the 10 year payment plan, and a column for additional payments, which could be any amount I chose varying from month to month, but I want to see how the additional payments change the remaining balance and the amount of payments left.
I have looked at many pre-made excel templates and online loan calculators/generators and cannot seem to find anything that will simply create a worksheet that will calculate the loan based on payments made and actual remainder amounts. I figured having the math worked out itself, instead of using the PMT function, was a good start. But if there is a better way to do it without the above formula, I'm all ears!

Does anyone have any bright ideas to accomplish my goals? Or does anyone know of a template that will function for what I want?