The thing the experts always say is not to charge by time, but by delivered value. I haven't got there yet, and my clients seem to prefer to work with a time-b

The thing the experts always say is not to charge by time, but by delivered value. I haven't got there yet, and my clients seem to prefer to work with a time-based rate. YMMV. But ... I do adjust my hours according to my productivity. So, if I need to study up on something, I generally won't charge the client for that (or not all of it) if it's reasonable that I should already know it. On the other hand, if I've been pumping out great code for the last 4 days solid, then I'll still charge a full day for the fifth day when I get a bit less done.