Nick,
5% is a very conservative figure for commercial ops. NEO mentions the Davtron (on the B-212) and I can vouch that typically the pilot flying time was about 30% greater than the recorded airborne (tech log) time. Interestingly, when we introduced the EC-155 (on which airborne time is logged by HUMS), this proportion changed even further to more like 60%! This was because the sector time went down by a third but the turnround times increased (harder to load baggage and pax simultaneously.) Obviously sector length and numbers are the key points and the longer and fewer the sectors, the smaller the difference.