Didnt, and still dont, have any app for this. Used the LIDO tables but a problem arises as you are doing 81/2 nm per minute as you are doing this. So the problem becomes WHERE will you be at sunset time? Took a good guess at the approximate wpt because it always goes dark around there when returning from europe.....LIDO corrected for longitude gave me an approximate sunset time and if that equates with your ETA at that waypoint then you have it cracked! If not, make a sensible decision to move back or forward a waypoint and try again....thanks for the altitude correction though, I guessed at about a 10 minute correction but nice to get it confirmed.
Having done all that the other night, my oppo, who was watching my mathematical shenanigans and also guessing somewhat wildly as to the time then produced his iphone app and pretty much confirmed what I had worked out!! Grrrrr!
Tried downloading an app today (doh, why didnt I think of that before?!!) and was unimpressed to find that I would need to upgrade to IOS 6 or something like that first, so gave up!! Far more satisfying to have worked it out for oneself....but there again, that sort of thing appeals to navigator types!!