The nitrate test was the one I was having the most trouble with actually. Has yours ever given a false negative? The first time I tested my nitrate (following the directions to the letter) it was 20, then the next day it said it was 0! I tested it three times with well-rinsed and dried tubes and I shook the living daylights out of that bottle number two, but it kept coming back yellow, which I knew was impossible. The next day it was 20 again. I never did figure that one out. I'm a little paranoid about the nitrate test reliability now. I know the AP kit is the "best" one out there right now though, and it hasn't given me any problems since.
Anyway, I think you're right about holding the tube slightly away from the card. Of course, the exact nitrate value isn't crucially important for most established tanks, but it can be when your tank is cycling and you're (not so) patiently waiting to get your fish! I wonder how many people have done water changes because they read the nitrate as a shade darker than it actually is