Hi freemp,

I would take the speed at 16 minutes as 400 because it meets the ASA criterion.

I personally would take the 16 minute time as "N" and then the different N-times are one unit different than what is programmed as N numbers in the spreadsheet.

Experiment with a sheet at 21 minutes, that may be your N+2 (my N+1).

As far as adjustments for batches go... It would be significant to a lab, but I wouldn't consider the variation you see significant for personal photographic use. I just keep my Time-CI chart updated with scatter plot points from occasional sensitometry test sheets that I put in with my everyday film runs. And if the Time-CI chart predicts my results fairly well, I continue to work off the older charts.

p.s. I graphed your data on paper, and my results are similar to the spreadsheet (except as mentioned, I call 16 minutes N)