I'm no examiner, but I'll add my tuppence;
The diversion does indeed need to accurate, but at destination.
This worked for me:
1) Draw line from current position to diversion. Mark half way point (this can be done at any point you get there)
2) Use a protractor thingy to get a rough mag track.
3) Add or subtract 0/5/10/15/20 degrees for drift accordingly. This is your heading. Tell the examiner.
4) Give them a time based on a rough calculation. This depends on what speed you cruise at, but find a rule of thumb that works.
5)Start clock and fly the heading untill the halfway point. Nice and relaxed.
6) At HWP, double your time and heading error (there will be one) and correct it all. Now you do have very accurate figures. Dead easy.
Example, you are 2 mins early at HWP. You will be 4 mins early at diversion.
You are 6 degrees left of track. Turn right 12 degrees.
Tell the examiner your new figures and they (should be) spot on.
So, from a rough bit of guestimation you later end up with some spot on figures. Bang on time. You will notice that the only thing you have to do accurately is work out the HWP. But you have ages in the cruise to do that before you get there.
And of course, between each of the points above, LOOKOUT!
Works for me. Open to much more experienced suggestions though.
EK