I called it dead because I didn't want to see a fielder attempt to reach it in haste and get his hand or arm torn up. Someone who's not in a hurry would have no problems getting the ball out from under the fence, but in the middle of a game? It's a safety issue, so I handled it that way.
The bummer of the sitch was that it was the 4th inning, and the home team was up by 15 with the away team at bat. This put a runner in scoring position, who ended up scoring to make it 17-3. Home team didn't get a run that inning, so we had to go another inning.
But hey, at least I go home with a clear conscience, knowing I didn't screw anyone over. Home team still creamed 'em.
