"When customer is watching" I stopped doing mobile work for this very reason, Whenever humanly possible I get the customer to drop their car off first thing and tell them I will call them later.
All the customer sees is the scanner go on the car and the code reset happening, they can't (or won't) see all the other work you have to do, time on UKAT , putting tools away, tidying up, paperwork etc. etc. Of course they expect us to have the tools of the trade the way they expect a plaster to have a float and a large bucket, why they feel they should not have to bear their proportion of the cost for this equipment beats me, I just find it easier to justify the bill if they do not watch everything I do to their car (and I make fewer mistakes if they are not breathing down my neck!).

Shaun