Trying to write a formula for amortization that takes place behind the scenes

I have original mortgage amounts, and the date they were taken out, and estimates of what interest rates were during each year. I need to to calculate an amortization behind the scenes and take the ending balance at the number of months that have passed since the loan was taken out. This will give me an estimate on what is currently owed on a mortgage. I have many amortization tables, and I can attached one to a tab in a spreadsheet, but I'm not sure if calling to values in that sheet is possible since I'll have about 50 records on each report that need to have that formula run in order to get the estimated current balance for each record. I can also have a table that can be called up for the various interest rates for each year that these mortgages were taken out.

My column titles will looking something like this:

Mortgage Date, Mortgage original amount, Estimated mortgage balance.

Does that make sense or have I confused everyone?

Thanks

I've figured out how to determine the number of months that have gone by, but what's im really having the problem with is how to take the loan amount and put it into the amortzation schedule, and then reference the ending balance for the given number of months. It's not that I can't create an amortization, but that I want to be able to use the amort. tool over and over in the same spreadsheet, to be able to send the numbers through over and over OR to be able to do it all in one formula without writing a separate amortization tool.

So I'll have all my mortgage amounts and dates, and the number of months into the loan in one tab, and the in the 2nd tab I could have an amortization table. I need to take that data and have it automatically use the amortization table, and then take data from it to go back to the 1st tab. This is what i'm really hung up on...