I've sold tons of these, and haven' had a ton of complaints on the fuel level, but then again, most of these guys are probably still in project car status lol.
Personally though, I have had a handful of Zs here that I've installed the gauges for them, and they all seem to read in quarters. The way I calibrate it is by removing the level all together, and just lifting it, and lowering it that way.
If you move the level out of the tank, it reads relatively smoothly, but for some reason when it goes back into the tank, it only reads in 1/4's.
On the flip side though, on my own personal Z, I was never able to actually fill it up to max capacity either (for some unknown reason). If memory serves, the factory tank is supposed to be 15 gallons or so, I could only get 8 out of mine, and I've actually let the car run out of fuel idling at the shop, just to test it. This was even when I first got the car, and it was relatively stock.