Help with Formula in Excel 2003

[Andy]

I have a column of data which I add to twice a day for one month.
I have a second column which calculates the daily average.
I need to calculate the difference between the last entry in the average
column & the first, placing the result into another cell.
What I can't find is how to make this a dynamic update.
Any ideas please?

 

[Don Guillett]

=AVERAGE(H2:OFFSET(H2,MATCH(9999,H:H),0))