With the difference between Hobbs and tacho, 15% is about right. The instructor often gets paid by the Hobbs hour and doesn't do many billable hours per day let's not forget that the majority of students expect the instuctor to give them a lot of unbillable time, eg hanging around after their lesson or turning up early wanting to chit chat, often depriving the instructor of a rostered break.
If the student wants to pay on tacho we give them that option, the price will be 15% higher because the costs are worked out at 115 billable hours per 100 maintenance hours.