I should have added that the benefit of the grey card is that you know it is zone V so reducing and increasing exposure as Jeffrey has indicated means that the yardstick zone is a known zone. Even in summer judging what zone the shadow your foliage is throwing isn't easy. Why introduce subjectivity when you have a grey card in constant light? You could use a north facing room lit by daylight around midday on an overcast but averagely bright day. Plenty of those days in the winter, I'd imagine