need help accessing other cells within a function
 

5 func string1
7 func string2
9 func
13 func string5
14 func
15 func string 9

I need func to return the usec time difference between the cell to its left
and a prev usec time from a cell above it. BUT the cell above it must be on
the
earliest row that also has a non-null string in the third collumn. Also,
if the current row has a empty third row then the func should return "".

PS I kinda have this working BUT it only works when I insert the func into 1
cell.
If I select that cell and copy, and then paste into a selection of
columns it
doesn't work. All cells return the same thing as the top cell in the
range :-(