If you progressively halving the embers needed, you actually get less value.
When there is no enhanced success campaign or when I have too many embers to spare (e.g. right after a lottery), I would just auto fill everything and go back to playing the game.
As for the mathematically-inclined:
When there is an enhanced great success chance campaign, I start with 1 ember, then increase by 1 each time I don’t get a 2x or 3x. Reset to 1 if I do. For 5th anniversary when there is 5x enhanced success rate, I will go even as far as double each time, i.e. 1, 2, 4, 8 etc. This is partially based on the Martingale betting system, which is of course theoretical only and doesn’t work perfectly because of the level cap.
It gets a bit more complicated when I approach level limits. But since I usually level a bunch of servants together, I can do the sequence across all of them: say servant A is hitting the limit and B still has room, I would start with 1 ember on A, 2 embers on B, 3 embers on B (2x success), 1 ember on A, 2 embers on B, etc