(Disclaimer: I'm no expert, but I've been reading a lot. )
You'll get different flavors from 60 and 20 than 40... if that's all I had, I'd probably sub something like a couple ounces of crystal 60 and 6 ounces of crystal 20. It won't be the same as 8 ounces of crystal 40, but it's not an unreasonable substitution, IMO.
What I've learned so far, though, is that it's not just a matter of the numbers; the darker crystal malt imparts different flavors altogether (raisin, plum, etc.) than the medium and light ones.
ok thanks guys - just wasn't sure of how big a difference it would make if I combined 20L and 60L... I guess it won't be the same, but I'm sure it won;t be bad beer!
I ended up having a few too many with some company this evening, so I am not brewing til tomorrow morning now... lol..