Just for clarification on how an ETP2/3 (in my case) is defined...
1. Use the wind at the altitude capability and half the distance from the ETP to the desired ETP airport to determine the ETP plot position, i.e. GSR and GSC. Having the appropriate MB charts onboard are a good reference, otherwise how would you know the winds along the ETP2/3 course at the required performance FL? It's a ballpark figure, but still as accurate as you can get. Accuracy still counts, right?
2. Use two or three engine TAS and FL as appropriate.
3. ETP, because of all the obvious variables, is just an estimated position, but it can be a valuable tool should it be necessary to know.
4. Never having really used it is not an excuse or for not plotting an ETP overwater.
Just my old-school, two cents after manually plotting and training them for years. Adjust your two-motor, ETOPS flying machines accordingly.