Yeah, you're welcome to take on that calculus. :)
One further point that occurred to me this morning is that to be visible at a great distance, the walls have to be brighter than the scattered sunlight that makes the sky blue. Otherwise they'll be invisible against that glow. It's the effect that makes a series of increasingly distant mountains look bluer and less distinct the further they are away. The nearer ones have most of the skyglow behind them; the more distant ones have more of it between them and you. You can see this looking at the moon in the daytime sky; the sunlit side is clearly visible, but the dark side is the color of the sky. And I seem to recall that scrith is pretty dark, and the sun is hitting it at an extremely low angle, so the walls are probably rather dark even when sunlit. Also, I am a bit worried that I remember the word "scrith". :)
Again, at night this isn't an issue. I imagine that most visual navigation on the Ringworld is done at night, when the nearer sunlit segments of the walls would stand out much better against the dark sky, and landmarks on more distant parts of the ring could be seen more clearly.