Magnetic compasses suffer from acceleration and turning errors, Bloggs. Using the compass, roll out of a 30 deg AoB turn on North. Now adjust to 340 deg. You can't, can you?? So don't try - keep the DI synch'd and use that instead. If it dies, turn using a stopwatch, Rate 1 and the Turn and Slip (or useless bl**dy turn co-ordinator if you're unlucky!!). That demo takes about 5 minutes maximum - after which, forget the theory, apply the practice!!!