FTE is completely dependent on what your process is all about. Each process will have different styles of calculating FTE.
Full-time Equivalent (FTE) is majorly calculated based on how much human power is getting saved. If a single person is taking 5 minutes to process a transaction, a robot is being replaced in his place and the time for processing the same transaction is calculated.
FTE = (number of transactions)*(time spent to complete one transaction).
Apart from this, cost vs benefit calculations need to be made to check if the savings are actually visible and is the process reliable and accurate to replace the manual process completely.
Based on all the variables that affect the process, FTE can be analyzed before and after using a Robot based on which savings can be calculated.