Big Surprise !
That's exactly what is done........
Input the Flex/De-rate and /or Assumed Temperature,
N1 or EPR is calculated and displayed,
Thrust levers or Autothrottle moved to achieve that N1 or EPR
Even in the good old days of steam driven clockwork aeroplanes, (Super VC10 and B747-100/200 spring to mind) these calculations were done from the books, the N1/EPR was bugged and the trusty F/E set the appropriate thrust.
Not too difficult.....
