After having verified once in your life that if you keep the grey card angled in a certain complicated way, which is not easy if you are alone, and without projecting any shadow on it with your arm or your lens, it gives you a result that is consistent with an incident light meter, my advice is to totally forget the grey card (for light metering purposes that is) and just use the incident light meter in all those situations where the grey card would be used*.
The entire procedure of using a grey card with the correct angle and without projecting a shadow on it is so slow and clumsy that I don't see an use for it in an outdoor situation. Incident light meters are "cheap", work better, and are much faster.
* It's easier to verify that the grey-card method is equivalent to the incident meter method in the shade, the typical EV12 @ ISO 100 situation. In this situation, I see that grey card and reflective metering of all kind, and incident metering, normally agree quite exactly. As soon as I go in the sun for the test I suppose the "glare" of the direct sun rays weights more and the angling of the flat surface of the grey card becomes critical, besides the additional possibility of an unnoticed shadow projection on the card, and the meters don't agree any more because of a method error.
** Let's say reflected-light meters are calibrated for 14% grey. Whatever you give them, they reproduce - given a certain "standard" printing method - a print which is 14% grey. If you give them an 18% grey card (lighter) they would tend to give you a "closer" reading than with a 14% grey card. But if you angle the grey card in such a way that you maximise its "glare", it's even worse
Considering that light meters are calibrated for 14% or so, I would expect the grey card should be used in a way that minimises glare, not that maximises it! The instructions given for grey cards instead want us to maximise glare on the grey card. I'm confused
Doesn't matter. Just ignore the grey card. Grey cards suck. Incident meters work