When my instructor asked me to divert, he asked me to do it straight away, not from a point say 5NM ahead
I always teach my students to divert from a known point. If your positoin right now is the middle of nowhere, then fly to a definite point and start your diversion from there. You might choose to use the time it takes you to get to that point to do your planning. On the other hand, if you are currently overhead a definite fix, then start your diversion immediately - or, if you're not comfortable doing that, orbit around your point while you plan. (Actually, a square is easier than an orbit, because you can do your planning on the straight parts of the square.)
Do people do it this rough freehand way or do they spend 30-60 seconds and make sure things are accurate??
I teach to do it freehand, for two reasons. Firstly, you might not have rulers and protractors with you - I certainly don't carry them on most flights. Secondly, accurately drawing and measuring tracks involves, IMHO, too much heads-in time - a subject which is particularly relevant given recent events in Goucesteshire, and even more so in poor vis (which is probably one of the most likely reasons you'll need to divert in real life). The lack of accuracy really shouldn't be a problem if you can confidently read from chart to ground and fix your position along the way, even just once or twice, and if you have line features to follow then even better.
But, as with so many aspects of flying, there is more than one way of doing it. For my CPL, I had to use a ruler and protractor for planning unplanned diversions. I found the workload very high, trying to meet the required level of accuracy of planning whilst still flying the aircraft, but it can be done with practice.
FFF
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