There you go..i made a user defined function by the name of getamount and here it how it works...
=getamount(Your last paid date,balance till,Number of months in service)
So for Instance for row-12 it will be
=getamount(E12,$C$1,D12)
Now at the end of next month when it will be 181 months for the 12th row..and balance date at c1 will be 01/20/09...it gives you your desired result what you are looking for that is 83.33
Here is user defined function and workbook for your reference...To make it work hit on enable macros while opening the workbook..as userdeifned function goes into the vb module of the workbook..
Saurabh...