I don't think using the whizwheel in the airplane is more dangerous than checking the T&P's as far as head down time.
The wind should still be in there and for the reason of diversion I teach to fly the airplane rudder only for the time it takes to figure out the heading/time.
I require max. accuracy(within reason) for the diversion calculations, if the weather is marginal VFR 5 degrees off track over 60 miles could make you miss the airport all together if you're flying over fairly featureless terrain. If the reason for diversion is mechanical you sure as hell want to be ending up over the airport not 5 miles left of it!
