Difficult to estimate as we know nothing about the condition of the rifle. 20 ft/lbs doesn't necessarily mean shit. The energy of the pellet, leaving the rifle, will drop off substantially as the range increases.
Best way to gauge it is to set a chronograph, at varying ranges, and measure the downrange power. If we estimate 3ft/lbs, in a head shot, to kill a rabbit then this is how you'll know your maximum range.
It's difficult to give you a rough answer as all rifles will shoot differently.