there is also a wide variaiton in the quality of whizz wheels. The one I used was great for actual nav but had too much movement between the wheel and the outer casing, so could lead to errors. Only afterwards I saw my instructor and it was a really close fit. You will also get differences depending on whether you are using wind up or wind down method. For the CPL I think they have to use wind down method, again ask the instructor.
When revising for my nav I made up several scenarios and did them using the whizz wheel, then cross checked against a piece of software, so I could see where any mistakes were.
Where are you based? any difficulties I am near York
Good luck