Can I use a white sheet of paper instead of the grey card for white balance calibration? I actually tried and it works not bad at all. I know the grey card is calibrated etc, but does it really change?
A purist would say it's not good enough because you can't guarantee the consistency of the white in the paper. However in my personal experience it's plenty good enough. I've used sheets of paper, bleached coffee filters, tops of Pringles cans (when they had a frosted top) and everything was fine. I've also used more expensive tools like the CBL Lens.
However, a word of caution: if your goal is color accuracy then it's not good enough. You need proper tools. If your goal is to get very close then it's perfectly fine to use paper. Remember that in many cases a white balance choice is an artistic choice more than anything else. And there are many reasons to choose something other than what is technically correct.
I use the back side of 3x5 cards all the time. The one reservation I have with paper is if it has writing on the back side, it can show through, and in strong lighting, sometimes it's easy to see through the paper, and especially in strong light, you can see the grain of the paper much easier. If you use paper, you should use several sheets of it to prevent colors coming through. I would also watch out for watermarks when you are using a piece of paper for a white-balance.