Well, I've just spent two hours experimenting and testing, and I like it!
@ Peabody: Thanks for the suggestion on JSGME. I'm going to relable what I've done and make several folders, one for Atlantic, Med and who knows what.
@ Zel: Special thanks for telling me what I needed to know. There is one thing though - it's the opposite of what you thought. The smaller number is more clear, larger is less. I don't know why, but there it is.
Here's what I found:
.02
.05
1
2
5
Any darker than 5 and you really start to lose the boat at periscope depth. I'm thinking I'll experiment with 20 meters and see how that looks, as that's about as deep as a plane should be able to spot you. Maybe 30, so the top of the conning tower will be at about 20. We'll see how it goes.