Hocus Eye. said:
A sheet of clean white photocopy paper will give you a white. For metering purposes a bit of grass or a standard dry tarmac road will give you an 18% reflectance grey as near as dammit. You could of course buy a Kodak grey card which has a white back, assuming these are still on sale in the days of digital cameras. You can use a sheet of white paper (or the white back of a Kodak grey card) in low light conditions to give you a reading if your meter is not sensitive enough. This then needs to be increased by 3 stops more exposure to get a standard reading.