1. Sane customers value determinism more than they value maximizing the productivity of a single hour. They're engaging you because they want a thing done, and the order of their concerns is: (i) assurance that the thing gets done, (ii) getting the thing on a predictable schedule, (iii) getting the thing done sooner, (iv) that they're not making a fool out of themselves by paying way over the market for the thing and (v) how much they pay. Note how far down the list the actual absolute billing number is.
2. Conduct a thought experiment: imagine you have a project that is going to take you a week to finish. Now imagine you take 100 Provigils (don't do this) and get the project done in 3 days instead. Who should get the economic benefit of that efficiency, you or your client?
You're metering, but at a lower level of granularity. You don't care about squeezing the maximum customer value out of ever hour, or maximizing hours you bill but don't do work in, because you're just not tracking hours. You work at an ordinary pace. Some days you're going to give your client 9 hours worth of productivity because you're in flow; sometimes they're going to get 7. It doesn't matter.
The only really important thing about your interval is that you can't book two clients on the same staffing in the same interval. If you bill daily, you as a general rule don't book two customers on the same day (really, I wouldn't do it in the same week).
