calculating payback of an investment
 

I'm trying to create a function that calculates the relative payback
period based on an investment's annual net revenue. Here's some
sample values for the net revenue of an investment each year:
Year Net Revenue Cumulative Net Rev.
1 -6 -6
2 -4 -10
3 -3 -13
4 -1 -14
5 1 -13
6 3 -10
7 3 -7
8 3 -4
9 3 -1
10 3 2
11 3 5

So, I know that I could do a simple function to return year in which
Cum Net Rev is greater than 0 (such as
SUM(countif(CumNetRevRange,"<0"),1), but what I'd prefer is to be
able to calculate the fraction of the year in which I break even. That
is, I know that in I break even once I realize 1/3 of my net revenue in
year 10. Assuming that my revenue's accumulate at a constant rate
over the course of the year, how can I write a function that, given the
data above, will yield 10.33 as the payback time?


I'm using Excel 2003, and tried to use the solution posted here
(http://groups.google.com/group/micro...0da517f9e562b4)
but have been unable to get it to work.

All help is appreciated!

Thanks,
Robert