What you need to do is go and talk to them. Ask them about whether they submitted the distance,how they measured it,and if you have to explain how you let them do these things on your property and that it would be common courtesy not to pursue a technicality for the shooting. If they are a holes about it, no more walkingand mountain biking on your trail. BTW, is the regulation 500 feet, and if so, is there a place on your property that IS 500 feet?

The main thing is that when you talk to someone, usually everything is fine.