I'm not sure if there's an optimal line length, but I can give my experience as a data point. I use six feet of line for my CO2 here in Denver and I'm running the same line on my nitro set up right now, and its pouring great.
And this is my first nitro beer that I'm serving. I knew as much as you know now going into it (keg the beer, let it sit at 30 - 35 psi of beer gas for ~1 week, serve). And it turned out perfect...easy as it can be.