My personal syndicate charge Hobbs time less 0.1 for every full stop landing. This encourages people to do a proper warm up, and not rush a back track for the next take-off.
My Coastguard Unit charge Hobbs as is, 'cos we want re-imbursement from a grateful public, but only log Hobbs less 0.1 per full stop for the maintenance log - ( this is legal ) cos we have to pay the engineering bills.
You can charge, or log, whichever you wish, organise it to suit yourself.
If only charging and logging airborne time, then an airspeed switch attached to the Hobbs it the way to go. Fair to all, then.