What accuracy to expect

Hi I’m looking for some advice on what accuracy I should expect/be aiming for with a calibrated pinhole camera.

If I scan a board of 6mm 4x4 Aruco images and estimate the distance between each of the points with a calibrated camera what sort of accuracy should I be getting? at the moment as an example if I set markers 15mm apart I’m seeing results +/- 2 or 3mm is this the expected accuracy with a single camera or should I look at my camera calibration?

Many thanks