Calcualting Interest and keeping a reducing balance

I am sorry if this is the wrong form but this is my first thread. Could someone point in the right direction to setup a spreadsheet that calculates interest on a capital sum but one where a reducing balance is shown against costs. Here is the problem. I need to setup a spreadsheet that startes with a capital sum invested at a variable rate that can be changed. The capital sum will generate income to pay for an old peoples home place for a relative. I want to work out at a given rate of interest how long the capital will last against the cost of the home.

Is there a tutorial I can access to see how I might set such a spreadsheet up?