Most crossbow scopes are calibrated for bows shooting in the 315 fps range, your scope probably has three crosshairs which should fall in nicely with a 20,30 and 40 yd sight in, set your top crosshair for 20 and see if this isn't the case...Excalibur makes a scope that is adjustable for your individual bows speed but most other scope makers use this 315 fps formula................Good luck and enjoy............Hillbilly.