+1 from me. I definitely agree that you have to start by tracking your time well. This is something I've always struggled with because I get a lot of calls every day for smaller tasks that I attend to immediately. Most days there is a 30-60 minute job that I started at least 4 times. Like yourself I do quite a bit of reviewing before I decide how much to charge. I ask myself questions like:
* What type of work was I doing?
* How important was it to the client?
* How long would it have taken if I had one contiguous block of time to devote to it?
* How long would they have expected the task to take?
* How long did I expect the task to take?
* Were there hold-ups because they didn't explain themselves clearly, or because I had to chase them?
* Were there hold-ups because I had to do some general learning to get the job done, or because I made a mistake?

I'm sure many people will say I'm over-thinking it, and perhaps I am. But it generally only takes a second to determine most of the above because its already in your head, and I like to be careful about what I charge. I don't want my company to be one that has a high rate then charges for every conceivable second. We're tripled our number of maintenance contracts in the past three years by providing good service and being reasonable about how we charge for it, and most of our new clients have all said the same thing when we signed them up... "We moved away from the old provider because they charge for everything, even to fix up their own mistakes, and they weren't clear about what would be charged in the beginning". (Actually there's a second common answer too - "They didn't fix our issues in a timely fashion").

Our most recent new client told us what tipped them over the edge. Their current provider sold telecom gear as well as doing IT work. They spoke to the client and said "Are you happy with your current phone plans? We'd like to do a quote for you on a new system". The client was happy with their phone system and didn't want the quote, but gave them the chance to quote anyway. The quote came in a few weeks later, and it was much more expensive than their current system so the client said no thanks. Later they received a bill in the mail for $1,200 for "consulting" on a new phone system!