50mm should be enough. I run 38c tyres under 45mm guards and it fits well. I used to run 28c tyres under the same guards and it looked a bit gappy. For the same reason, I think 42s under 60mm guards might look a bit off unless they're really knobbly tyres. Both should work fine though.
Looking at getting some GB guards for my crosscheck running 700x42 tyres and not sure if I should get 50mm or 60mm?
Thoughts anyone?