powdermonkey's method is exactly the same as mine. It's important to remain calm and break it down into easily manageable chunks so you don't get overwhelmed and you can keep everything really simple, then just practice and the nav gets easier and easier.
For diversions, as has been said, you can worry about the exact calculations once you're on track, so make the turn early with adjustments for drift. Once you're settled on track and you're straight and level then you can worry about GS and ETA etc.
With practice you can do a complete diversion plan, from finding out where you're going to informing the instructor of the plan, in under 90 seconds. Then you can relax a bit until you're immediately asked to do another one
