IceFire, you're thinking damage done in based on distance from impact / detonation; we're talking about damage done based on distance between point of origin and point of impact.
I'd go with a geometric reduction approach instead of an arithmetic reduction approach. IE decrease by X percentage of the remaining damage amount, so that a -10 100 falloff doesn't leave a 100 damage weapon doing 80 at 200 meters, but 81. This way, the weapon will still do damage upon impact as long as its within range (the damage could be infinitely small, but you'd still do damage).