Idaho Fish and Game does not have a code or rule that specifically defines the distance you are required to be away from the road before discharging a firearm. However, Idaho Code 36-1508 (copied and pasted below) just explains that you cannot shoot from or across a public highway. If you are off the traveled portion of the road and you are not shooting across the road you are fine.
FISH AND GAME
36-1508.Shooting from public highway. No person shall:
(a) Shoot from Public Highway. Discharge any firearm from or across a public highway.