As buster says, if you make a mistake just keep going! It may not be a fail but if you dwell on it the rest of the test will go to pot.
Also I dont know about you but when I'm under stressful situations (CPL, IR etc) my mental arithmetic slows down. I found it really useful to have some pre-calculated times for my diversion based on the day's winds e.g. 10nm will take me x mins etc. So I could provide an ETA quickly and concentrate properly on the diversion.
Oh and dont do it in a 40 degree heat in a plane with no air con.....
