if/then calculation on indirect cell locations

Greetings: The following programming need has me flummoxed and hopefully someone out there can help me. While I have some experience with Excel, I have none whatsoever in Access.

An example dataset is attached. Our data are in multiple files...each one contains 65K lines of data (we have 50 or so files). My data manipulation needs are pretty basic....check to see if a location was successful (column D), and if so, calculate the mean activity count for the 6 minutes surrounding the time of the location fix (3 minutes prior and 3 minutes after). Place the calculated mean value in an empty cell that is on the same row as the location point. Check the entire file. Perhaps make it possible to vary the mean calculation, adjusting for more or fewer than 6 rows.

I am stymied by the need to identify the row number and then use that to direct the row span needed for calculating the mean. I suspect it is a simple fix that is just eluding me.
thanks for any assistance

Alces

Duh, I figured it out. Never mind