Looking up the matches of each cell with the following cells in the same column

I am working on a large database of a water treatment experiment. I have a column with the cumulative value of a variable that I'm trying to find the matches based on a specific value. I am trying to write a code that starts from the first cell of the cumulative variable and goes down and subtract the value of that cell with the following ones and when the subtract value gets to that specific value (in my case say 25) or the closes one; index the position of that cell.

I really appreciate it if anybody got an idea of how could it be done in R.

Thanks