Hey,
I too had problems with Nav. Failed the first time, passed second time in 90's. I posted a thing on here about it entitled "ATPL Navigation question" and "antenna polarisation".
The responses I got from these posts were very helpful, and quite pertinent to the CASA exams. With regards to finding the True Altitude, unfortunately you have to be quite accurate to achieve the correct answer; this too applies to PSDs (I thought what I was doing was wrong, when in fact I was just being inaccurate). If you have any specific queries I'll do my best to help as I jotted down a few notes from the actual exam (that is, as soon as I get this air law exam done tomorrow).