Greeny - it is best done by 'puter - normally a flight planning programme is used. If you are trying to 'fag packet' it, good luck! Essentially it is a series of iterations for successive points, at each one of which your 'GS out' and 'GS back' change due to wind changes. I guess you could write a spreadsheet construct to help, but ...............
You would need to stick a pin somewhere 'sensible' on your route (ie start with the still air ETP and move it a sensible bit along/back on track just eye-balling the wind chart) and work along and back to see what happens to flight times as you move it.
What is your actual need? It can happen in flight with a re-route and then, well, you just do it to the best of your ability.
We do not carry the software (yet) to enable a new ETP structure to be made in flight.