In reply to DaveMinsh: I bought my prescription sunglasses from Optilabs. Emailed them and they sorted me out with a darker tint suitable for high altitude mountaineering. They offered a top rate service sending out several samples of frames to try with return postage paid. Then I just emailed them my prescription, paid and my glasses were delivered to my door.
I do think that how dark you want your glasses is down to your own choice. I have been on trips where people were wearing very dark goggles and still getting headaches from the sun while others were fine with there much lighter shades on. I find wearing a baseball cap under your helmey is a big help in keeping down the glare.
Enjoy your trip.