The further from the lines, the better. Don't think that you're OK if using Dacron lines. Any moisture, even from the moisture absorbed from the air on a humid day can make them conductive. My father lost a childhood freing flying a kite with cotton string too close to 220 KV lines.

There are guidlines about how far a given Voltage will jump on a dry, windless day. For example from my own experience, 460 Volts should only jump something like 2.5 inches. After an experience, my version is 18 inches. I had the bruises on my back for a while from being thrown against a piece of equipment when the Voltage jumped that kind of gap through an empty fuse box and open contactor.

I'd say that the closest the runway for a CL plane should be to the plane of the lines on the ground should be at least twice the length of the lines you're using. An add any fudge factor you feel comfortable with.

I've only been zapped once while flying CL, and that was on a hot, dry but overcast day in North Africa. Cost me a plane and engine.