Last year I was on a no-questions, no deviating day-rate. The consultancy I was working through wanted hourly accounting, I told them no and charged my rate per day or part of a day I worked for them, and based my working practices around 7.5 hour days. I put in the odd extra hour or ten here and there because I liked them and enjoyed the project, and they never quibbled about me clocking off early when the project didn't require a full day's attention. Quid pro quo I guess.
For this current contract the agent demands an account of hours worked and pays extra hours at day-rate/7.5, up until some maximum at which time I get to charge double, but the time has to be authorised by the end-client.
But it sounds like you're doing much smaller chunks of stuff for multiple clients? I tend to dedicate myself to one client at a time and work more or less full time on one project. Serial monogamy one might call it... I'm not an employee because I work on a project basis and then leave with no fuss :) I'm definitely a contractor, are you more of a freelancer?