The OP is hiring an aircraft and paying via the Tacho counter. He is asking if they should be logging the flight time (personal log) via the Tacho or chock to chock. I explained clearly that Tacho is a maintenance counter which generally under-reads block time. Chock to Chock is most accurately represented by VDO, if you don't actually record it via watch times, which is perfectly acceptable, we do that in airline operations. The other option to Tacho counter for maintenance is Air switch with a factor for engine time. As for airframe maintenance airborne time (airswitch) is all that matters in most cases. In airline ops you just manually enter watch time off and on block for personal logbook, and the airborne and landing times for maintenance. Then the smart guys in tech do fancy calculations based on how it flew as to what cycles and hours remain.