I think you really have 2 questions to answer. The first is about the level of light that will damage your eyesight, and the second is about how dark is comfortable for you. the second question has a different answer for each person, as there is definitely a difference in light sensitivity from one person to the next. I, for instance, like very dark lenses. I'm the first guy to put on shades while others are doing fine without. I have a pair of Julbo Colorado glacier glasses, 5% transmission, and I often with they were darker. I use them year round, with the side shields for backcountry skiing, and without for backpacking in the summer. At 12,000 feet in early May on the snow, I wish they were darker. In August, on the dirt and granite, they are about right, and I use them at elevations down to about 6000 feet just fine. Your eyes my be more or less sensitive than mine, so your needs may vary. I tried some photocromic Julbos. which adjusted from something like 7 to 3 or 4%, but they did not fit me as well. I can defintely recommend the Colorados as a good value, only about $50 for a quality pair of glasses. I would suggest that any pair of glasses for which you can't find out the light transmission should be ruled out right away. Don't take any chances with your eyes. Better to get a pair of glasses that may turn out too dark for you than the other way around.