How to get row information for Google Sheets Updater

I am reading from Google Sheets, finding the first row where a certain cell is blank, processing with information from the other cells in that row, then writing back the missing cell. The Google Sheets Updater wants me to specify the range in A1 notation. What’s the best way to capture that information when I find the first row with a blank cell? Or should I use something other than the Google Sheets Updater?