Prefacing expectations. I state right at the beginning of any commission that I allow only 3 revisions, once at every "pass" that I provide.
Anything extra will cost extra, and that cost is high. This is to temper the nitpickers, or wishy-washers, because honestly the money we take in is not worth the hours and hours of revisions. It can really get out of hand.
Yes, I do want people to be happy with the work I provide them, but I have no time nor patience for small adjustments that can creep up time and time again.
This is only a slippery slope if you let it be. Personally I don't allow my customers to run-on with minuscule changes and I let them know that frankly right from the get-go.
It's a hard rule that I try very hard to stick to, but I'll only break it when it has to do with placement or measurement changes.
This is why I tell people that when they give me exact measurements at the beginning, and the revisions feedback must be well-thought-out. Take your time to think about what you want changed and at each pass try to give me as much feedback as possible. I realize this is hard for most people who are not creative-types, since they have a harder time envision what they want. But that's the rub eh? When you are hiring an artist for a commission, it's up to the artists interpretation to deliver what you think you want. This is very similar to how tattoo artists work...you go in, give them an idea of what you want, they show you examples, or give you a rough sketch...you sign off on it and it's direct to the inking.
As for the starting point, my customers must provide me with as much information as they can before I price out how much it costs. I have a "standard workload" that I let them know about. Creating 40+ characters collage doesn't not fit within a that standard workload.
Anything above the standard and I scale the amount of time and money it'll cost drastically. This keeps people from going crazy, and if it turns people away from hiring me, then it's no skin off my back. Like I said, the money and time to do these commissions are not worth that kind of effort. I do this for a little money in my pocket to fund my hobbies, so the wife doesn't go ape----steaming pile of meadow muffin---, as well as a service to the community 'cause I like seeing nice art'ed-up cabinets being made. If it comes to a point where I feel its not worth it, I might as well just make art for myself and say screw-it, people can stick to that fugly blue lighting stock art.