I got 74% and I've been doing a 2 year course as well but I was doing national 4 in fourth year (due to my lack of interest in maths) and moved up to national 5 in fifth year, I have done my prelim and it was fairly challenging but not to worry, the grade boundaries may still lower depending on how people do that is...
My advice would be to everyone, to know exactly your enemy. Mines right now is these bearing ones, and recognising these sine and cosine rules, but I have so far done really well since I have went over it..
Also, remember you still get marks for going the right way about it, irrespective of the numbers.. So what you won't get 3/3 or 4/4, atleast you still get something !