I would recommend the standard dark grey issues in any form of sun.
Too dark glasses are to be avoided in flying as they distort colors. Only when you overfly the icecap at low altitudes or when you fly at very high altitudes (e.g. above 45.000 ft ) do you need extra dark glasses. But then a helmet with 2 sun visors ( i.e in heli or jets ops ) are generally the best. not sunglasses .