Is there some rhyme or reason why 20 feet from the transformer would be a good place to put a choke?
The near-field of an antenna will couple to
any nearby conductive objects or wires, including the feedline, if they are not centered in a balanced field. This happens whether the wire or feed line is connected or not to the antenna (as it does for unconnected Yagi-Uda elements).
So it's best to put a choke balun past the point where most of the near-field RF is already coupled into the feedline, a bit farther from the antenna. If you put a choke before the RF is coupled into the feedline shield, near the antenna, it can't attenuate that coupled RF. Where the RF near field ends depends on the antenna geometry and size.