Same here Gen Nav was my bug bear.
And to this day I still reckon i didn't know any more between the time I failed it and passed it.
Don't know if the format has changed but there used to be 1 killer question at the end that if you got it wrong you could only miss another 4 questions or 2 if it was the 2 mark ones and that was you stuffed.
I used to think it was all pants but I have spoken to a few pilots that had to get the tools out when things have gone wrong and although rusty they managed to get through.
Alot of it seems pants especially in the UK but when your GPS drops out somewhere near greenland in a turbo prop with 3 hours fuel left and all you have is 2 NDBs to do your fixes by, huge amounts of variation. The do I add it or subtract it becomes pretty vital.
If I ever get to the stage though that i am relying on my knowledge of compass turn errors. I am knackard. It used to take much waving around of a CRP5 and a pencil to work out what was going on.
MJ