finding year associated with last negative value and then calculating value

Hi,

I have a table that looks as follows:

i j k l m etc
Years 1 2 3 4 5
row 69 2600 2400 3000 3000 3000
row 71 -10000 -7400 -5000 -2000 1000


Row 69 has cash generated
Row 71 has cumulative cash position at the end of that year

the formula should find the year associated with the last negative value in row71 and add that to a fraction of the cumulative cash position in that year + the cash generated in the following year. In this example it would be 4+(-L71/M69) = 4.67

would this use a hlookup function?

Thx

I have attached a sample file. thanks.