I've posted the following several times - reposted for your interest:
I have a pair of Varilux type prescription specs which I had made with a graduated neutral tint of 75% reduction (#2) from about the top third rapidly graduating to fully clear on the med/short range portion and they work very well. The tint seems at first not to block much light but for traffic detection it should not be any denser. I've just checked them and a pair of company issue RayBans with my camera exposure meter and they both give a 2 stop reduction; I think that's 75%
The following is an interesting article which I have in a flight safety magazine but cannot find on the web:
Pilots' sunglasses: mystique or mandate? By: Dully FE, Jr.
Formerly Field Associate Professor of Aviation Safety, Institute of Safety & Systems management, University of Southern California.
The salient points are as follows: (Mr Dully's views, not mine)
1. Use glass or polycarbonate; not soft plastic which does not block UV.
2. For traffic/target detection: no darker than number 2 which blocks 70% of light; i.e. transmits 30%.
3. Wearing Brown #2 a 20/20 (UK 6/6) remains almost 20/20. Looking for traffic, the pilot should remove them. A US Naval report found that tinted visor wearers were at a 1.8nm disadvantage in target acquisition!
4. Green or grey are said to give the least colour distortion.
Brown will enhance contrast and block some blue.
Yellow #1 (i.e. colour filters, not sunglasses) can enhance visibility in low level poor vis caused by smog.
5. As soon a light levels/glare, drops remove sunglasses.
6. Photochromatic lenses are not suitable for aviation use nor are polarisers.