I have a depreciation project and I am trying to find a formula that will
result in the last year's amount. Using straight line depreciation, I have a
formula that removes 1 month of depreciation in the first year. The formula
I need should give one of two results, either a full year's worth of
depreciation or the remaining amount in the last year.

This is the formula I have so far:
IF(C18SLN($F$5,$F$6,$F$7),""<SLN($F$5,$F$6,$F$7)) ,IF(C18<SLN($F$5,$F$6,$F$7*12))

Can anyone help?