PBzeer, I've looked all over for charts that show the line clearance, but can't find any. This isn't a Corps of Engineers lake, so that link didn't help. This lake (Lay Lake) also isn't a navigable waterway, so about the only maps you can get are devoted to fishing, and are just topographical maps with depths marked in 5-10' increments - not real marine charts at all.
Sailingdog, I also just discovered that transmission lines also sag when they are under load, due to the heat generated. So, I think I'll take the minimum required clearance and double it, just to be safe.
I like Boasun's suggestion. I think I'll get a slingshot, some fishing line, and a lead fishing weight and just measure the height myself. Is there any fishing line that will conduct electricity???? Or are they all pretty much safe?