Mine goes down to 2 bars minimum and when it reaches 2 bars it's a matter of seconds, maybe minute or 2 even with the car pretty much dormant, with no AC, not moving at all, before ICE will kick in. I'm not sure about the top, usually it happens when I'm driving at higher speed, often downhill, so I don't have much time to look at the screen, but it may be as well stopping one bar before top.
Question to Ken: is this graph for original Prius, or second generation? And is there a difference?
I read some more about usable SOC and I get conflicting numbers, from as little as 30% range to 40 % of total capacity.
Looking at standard NIMH discharge cycle one thing is obvious: it would be very difficult to predict SOC at any moment by reading it's voltage and it's the only simple way to do so. NIMH cell will drop of voltage very quickly being discharged from full, then it will level off and stay almost level for most of it's discharge time and then it will drop off very quickly to zero charge at the very end. That's probably main reason why ICE will stay on and keep charging battery to full sometimes, to recalibrate computer and then track SOC by calculating how much energy was charged and discharged, instead of just measuring voltage. That would solve the problem of voltage changing due to temperature and age of the pack.