I don’t think that is how field aid works. As far as I know field aid always heals the same amount. I think it’s 20% of the target’s HP. As for it’s timing it goes something like this (I don’t know the exact numbers, but I guess I’m pretty close):
5 defenders alive … field aid takes 7 turns to fill-up
4 defenders alive … field aid takes 6 turns to fill-up
3 defenders alive … field aid takes 4 turns to fill-up
2 defenders alive … field aid takes 3 turns to fill-up
1 defender alive … field aid takes 2 turns to fill-up
Someone correct me if I’m wrong, please