I've had good experiences with something like "$90 first 30 hours, $60 thereafter". You have incentive to finish work fast because you want to start new project

I've had good experiences with something like "$90 first 30 hours, $60 thereafter". You have incentive to finish work fast because you want to start new projects to get the higher hourly rate, at the same time, the tail hourly rate means they won't be changing requirements too much. When you must estimate a timeframe, make a honest estimate, and multiply it by 4.