Help with finding difference in current and previous row item values

Hi Forms Development Team,

We are using Forms 11.1.2.2 on Linux Platform.

Here is what we have a requirement of the new column highlighted by me in the attached image. We have added the Exposure Time Diff column and exposed in Data Block and in respective Canvas.

You can see that the first row of this column will always be blank and then the second row will be the exposure time of the second row minus the exposure time of the first row (2.25 – 2.00), and the same moving down the rest of the rows.

Please help with the best approach to meet this requirement.