Many of the scopes I have installed for crossbows regularly indicate that after you sight in the top crosshairs for either 20 or 25 yards then subsequent lower crosshairs may be on a variety of yardages within a given range. For example, the second crosshair may be dead on at 27-33 yards and not exactly 30...and so on and so forth.

Just for argument's sake though, yes, I would guess that the subsequent lower crosshairs should be about 10yards apart and, yes, you should start at 20.