[JonathanH]

Most importantly, you also have to figure out whether all this is worth your time. For example, I've learned to be pretty relaxed about charging: I tend to charge my Kindle Touch when it gets down to 25% or so (eyeballing from the gauge), and unplug it again when I notice the charging LED has switched from amber to green. So I'm doing 25%-100% cycles, instead of teasonc's 40%-80%.

Maybe as a result of this my Kindle's battery will "only" last 600 charging cycles, whereas teasonc's will last many more (shorter) charging cycles. Since I charge every couple of weeks, that means mine will "only" last for.. let's see... about 24 years. By which point I'm expecting to be reading on something bigger, lighter, and cheaper anyway. And if I'm actually still using it then, I'll have saved myself 600 instances of worrying about the "optimal" charging cycle. A bargain!

[There are many caveats here. A 24-year-old battery will degrade just from being old. I plucked the number 600 from out of a hat, but 500-1000 cycles is a good guess. A lot depends on Amazon's battery calibration parameters, which we can't change - cf the current story that the iPad3's parameters are buggy, and its "100%" figure is actually set too low]

Edit: I think we can come up with a reasonable rule of thumb here. That is, you should only worry about this stuff if you use your Kindle so much that you have to charge it more than once a week. If that doesn't apply to you, then don't worry, because your battery will die of old age (10 years) before it it dies of too many charging cycles (500-1000 cycles).