I think if the engineers who design high tension power lines feel that the distance from the ground up to the lines is far enough to offer a good safety factor for discharge, this distance from the model to the lines should also be enough. If you look at hi-lines you will see that the distance across the insulators that suspend the lines below the cross arms is no more than 6 feet at best and the lines don't short out to the tower. The AMA's 50 feet should be safe as long as it is always maintained while flying.
But saying that, what do the engineers know! I would double the distance from the ground to the line just to be safe.

Orv.