When I was a CFI, I undercharged (like just about everyone else) at first. It took me a while to realize that I was providing a service, and no matter what, the client is paying for that service/professionalism the entire time. I was more "fair" later on. I didn't charge for pre-flight when I wasn't with the student (so it was assumed they would pre-flight on their own after the first few introductory flights), but as soon as I was there at the airplane doing MY checks, the clock was ticking again (considering pre-briefing). If we had to wait for some issue, the clock was ticking. Let me ask, WHO is responsible for that student's safety while you are with them? It's resting on YOU, and I have had students with ladders about to try and move around the front of the aircraft while another was taxiing past, so as far as I'm concerned, it's the entire time you are with that student, and that's being "nice", since I'm still required to be there while someone does preflight and if they screw up, it's STILL my butt.
I'd say ground time was averaging .7-.8, significantly more for those first few lessons where you teach preflight, not a whole lot less than the .8 figure though, that's usually what it was. Sometimes the average with CFI-students was a bit more, as I'd go into more depth with them, so more like maybe 1hr. Think about all the things you are really supposed to check for each flight, what is really required by regulations, aircraft condition, and of course pre and de-brief.
I would ALWAYS encourage my students to preflight EFFICIENCTLY and I'd try to give as much help as possible to do this, but if I had the student that decided to give the aircraft an "annual" with a 45 minute preflight, I'd be standing out there after a little more than 20 minutes and I'd make him pay for that extra time it took to preflight. This would usually be after disussing it a few times, and I would NEVER tell them to "speed it up" while they are doing it, but I also have other students have to try and schedule to utilize my time efficiently.
Now, I wouldn't charge for time that was not spent towards completion of the unit, like lunch, or make any "special deals" (I'll charge you x amount less if you buy me lunch) and so on. There are many ways to be unethical, and most likely CFIs are fairly young without a solid foundation of such things, but most do ok, if usually undercharging. Just don't do any stupid stuff that will get you fired from your flight school or do any of the same stuff on your OWN that would have the same result IF you worked for a flight school. Sometimes they don't put out a whole lot of guidance saying what things to not do, but that doesn't mean you should do them.
Some instructors actually resort to using a stop-watch. It's not a bad idea.