Personally, I don't like a flat emergency charge plus standard rates because time on site is just too unpredictable. I've had too many situations where the disruption to my planned schedule exceeded the value of an emergency fee plus regular rates. I think you have to think of your costs in terms of inconvenience to other customers and a potential loss of good will. Sometimes, you can lose a job (or even a customer) over an issue like this. But it's definitely like walking a tightrope sometimes.