For the exam use what he instructor sugests. But in my real world captain custard shows the way. It is also easy to visualise what you are doing. As for the 10% additional fuel taken out of the calc as you reach your PNR. you can re calc closer to your PNR or use some of that fuel as your fudge factor that will be used during the emergency decent for dep cruise if required. I've never experienced one for real, but in the sim everything seams to take a moment or two before bearings are regained . Depending on speed it will also take some minutes to do the 180 turn, while burning critical fuel.