I have a per hour fee, but after listening to the project, if possible knowing the level of the musician(s) involved, the length and complexity of the songs, and all other aspects (I like to go very personal, maybe attending rehearsals/shows or at least socializing and hearing any material they have) I try to make an estimate. I already know how long I take on a 4 minute song when editing, programming, and the other parts depend more on the band. If I´m unsure how much the tracking process can take, I try to give the band a daily fee to keep it less "strict" on the hour thing, mainly just to have a bit more relaxed atmosphere during recording, I don´t want to stress them by having to look at their watch all the time.
your hourly/daily fee should depend on you and what you think you´re worth (and the reputation you have built within the scene, if any), but you need to have a minimum. At first I did 10 euros an hour for programming/editing/tracking thinking that in my day job I get paid 8 euros an hour, so I could say it´s kinda worth it, but my wife opened my eyes on this. In a day job in any company, you´re not paying social security, taxes, etc. while in a freelance job such as a recording studio you do (considering you are trying to do it for a living, and that is the main goal of many of us I assume), so you have to consider those 10 euros are actually much less after taxes.
And, you can´t really give a supermarket/flipping burger day job the same worth of recording/mixing/mastering. There´s this story, about a mechanic who got called to a company cause a machine wasn´t working. The guy went there, checked the machine, detailed for a while, until he grabbed his wrench, twisted a screw just a little bit, turned it on and the machine was working again. When asked how much for his services, he said 500 $, and of course the company president was shocked, and demanded him to give him an explanation on why twisting a screw is worth 500$. The mechanic answered "twisting the screw is free, I won´t charge you for that. Knowing which screw to turn, however, is worth 500$."
Just my two cents of course, and it´s not like I´ve been in this business for years