Sunglasses - how much light should be blocked?

[jma]

Just ordering a new pair of prescription sunglasses. All going well (apart from the cost.........) & then I get stuck on how dark they should be!

I've heard that 75 - 90 % block is best for 'general comfort' but 70% max for traffic detection.

I guess there is no definitive answer to what must ultimately be a compromise but has anyone received professional advice on this before I guess & spend a lot of money?

Thanks