Originally posted by AstroDave While an 18% gray card may be marginally better, you can use a plain old white sheet of paper to do a custom white balance. Check your manual on how to do this, but once you get the hang of it, it is very quick and easy. Just hang that sheet of paper out in the light situation you have and take the calibration shot. I ALWAYS do this if I have a question about the light source.
I have a white, grey and black card in my bag, they are actually samples of finishes but the grey is 18% and they are metal so durable . I actually got them years back someone had posted a thread here on the company providing samples of the finishes by request. I'm sure they got overwhelmed for a while on the three finishes with no requisite increase in orders lol