I've always been charged tach time.
It works out (generally) 1 hour on the tacho is 1.1 hours brakes on to brakes off. All my instructors charged BO to BO.
So your log book gets 1.1 hours and you pay for 1 A/C hour and 1.1 instructor hour. We were on a farm strip so very little waiting about on the ground.
CC