I've found that by far the best strategy has been coming up with a flat quote for the rough draft, and then billing the revisions phase by the hour.
This way, the incentives line up: the part that you have total control over, you are incentivized to go as fast as you can, to work as efficiently as you can, etc. Then once you've delivered a rough draft, the client is incentivized to be as clear as possible and not to endlessly change his mind. And even if they do, you're happy because you can bill for more hours.
I think splitting the difference in this way has worked very well for me in several years of client work.
This way, the incentives line up: the part that you have total control over, you are incentivized to go as fast as you can, to work as efficiently as you can, etc. Then once you've delivered a rough draft, the client is incentivized to be as clear as possible and not to endlessly change his mind. And even if they do, you're happy because you can bill for more hours.
I think splitting the difference in this way has worked very well for me in several years of client work.