The OPT ALT calculation is quite complex. It requires the entry of forecast winds along the entire route and at levels above and below your planned cruising level. For instance if you are planned to fly at FL350 then enter the winds from say, FL330 upwards to at least the maximum level you intend to achieve en-route. It is also important to enter the forecast temperature at the level you plan to cross each waypoint. Not doing this can lead to the situation where the OPT ALT was FL380 and you climb only to find that because of the new temperature at that level the OPT has reduced to FL360.