We pay for tacho time in our PA28 group. Our tacho (and I thoght they all did) reads 1 unit for an hour spent at FULL power, e.g. red-line RPM. The advantage with charging per tacho time is that it encourages pilots to come back to an economical cruise, saving fuel and wear 'n' tear.
There are 2 ways of working out your log-book time, neither of which work for either tacho or hobbs.
1. Actually record the time you start moving from the parked position until you park up again.
2. Record your take-off and landing time. Add either 5 mins or 6 mins either side of this time (depending upon whether you choose to keep a 'minutes' or 'decimal' log-book). The advantage of this method is that it ties in with the requirement to record the airborne elapsed time in the aeroplane's technical log.
Both methods are approved, but require manual noting of the times. I've never seen an automatic method for either.
Our flying school, in common with many others, charges for the airborne time plus 12 mins. This has the adavantage for the early student that the 15+ mins often spent explaining the details of how to do taxi checks, power checks etc is free after the 1st 6 mins!
Cheers,
TheOddOne