Using last cell in a column that is not empty in a calculation
 

I'm using OpenOffice Calc 2.0. I don't know if that is the problem, but all
attempts I've made from other posts to do this results in #NAME or #VALUE?.

My data:
Col1 Col2
Jan 06 $300.00
Feb 06 $292.00
Mar 06 $330.00
Apr 06 EMPTY
..
..
..
Dec 06 EMPTY

YTD Difference Cell
Col2:3 - Col2:1

How do I get Col2:3 and then next month, after I enter a value into April,
Col2:4 - Col2:1?

I hope this was clear.