I've measured vandate based 532 at 1.2 nm wide. The 3Db or "Half Width" points were about .6 nm apart. If your looking at it by eye, you could not tell when you calibrated your grating.
A common neon lamp from Radio Shack gives you plenty of well known glow discharge lines which are a tiny fraction of a nanometer wide, and for two bucks plus a line cord and some splices to make a packaged neon lamp safe, how could you go wrong?