the whizz wheel should get you to within a couple of knots of the right answer, but the angular answer should be within two degrees also - if you are getting three degree errors, you should check your technique. One idea might be to ask another student to work through the problem with you.
As you say, for answering multiple choice questions, your technique will get you to the correct answer reliably. More importantly, your technique will also suffice for 'real-world' VFR flying as the wind will rarely be exactly as forecast. Your technique is more accurate than the weather forecaster's!