As shipped (default), dithering units in ACP are in guider pixels, so it's easy to tell how far it will move. If you put the guider plate scale in arcsec/pic into ACP then it will dither in main image pixels. Some people prefer this. The number you give is a sort of an average, but each dither is random in X and Y. If you put lets say 5 in there, it will dither up to maybe 8-10 pixels, which is way smaller than the guide sensor dimensions.

You can get the guider plate scale given the guider's pixel size in microns and the focal length (use the focal length that is measured, taken from an ACP observing log), then going to this online calculator: