If a school is charging tachometer time plus 0.2, then I would go to another school. In my experience, tach time is about the same as bLock time so you will probably be paying for more time than the block to block time you should be using in your personal log.
Our school charges for airborne time ( measured by a timer attached to an air switch) plus 0.2 to cover all taxying. This is a great system since it does not encourage students to save money by taxying too fast or skimping on the warm up. Students often pay for less time than they log, especially on the early lessons. It's fair for the school too, because most of their costs are based on airborne time.