Auto levels and long distance readings, accurate?

I've got a Wild NA1 auto level with 24x optics and I'm able to read the rod at 500' and still see down to 0.01' when the heat waves aren't too bad, but I don't have the manual and I can't find one online either. Does anyone know what this level is rated for as far as reading at long distances? My Leica Sprinter digital level says it's only good for 100 meters, but the optics also aren't nearly as good as the Wild. The project I'm working on requires me to mark elevations from a distance of 500'+, but I feel like I'm pushing it with this equipment.