the way I see it, you only have 1 shot at the first impression. jumping the gun and having something unbalanced, buggy and/or unstable will pursue it after its polished, limiting the reach of the game.
so in effect, the game is "done" when the game is complete, the balancing is reasonable, it is stable and there are no major bugs. perfection is not required at that point but you should already be proud of the product you put out, and make sure that what you promised would be in it..is actually in it. if for whatever reason you would have to drop support, the game should still be able to effectively stand on its own.
I am not really worried that you will choose the right time to pull the trigger, and even though I am not a backer and don't play the beta builds I'm fairly certain the final product will be a joy to play. I'm looking forwards to see the evolution of the xenonauts franchise.