Cat Driver
My point is that the only reason most people do this sort of navigation is because they are in training. Once you've got your CPL (this is in the UK) you wont bother with this again. You either become an airline pilot or use your GPS, VOR etc.
I believe for the CPL you need to arrive at your turning points within 2 mins of estimate. The legs are of the order of 50-100 miles. I guess you can correct your estimate at half way. I'm not sure at what point you can no longer adjust your eta without failing.
Clearly you would never climb in the overhead for any real-world reason.