It wasn't a 15 zhang attack. When XF first was preparing for his attack, he already started moving from his initial 155 feet (ideally speaking) distance. When the energy released from his palm, he was already 75 feet (ideally speaking). Before his energy dissolved, he released a second palm energy from 35 feet (ideally speaking). So that is actually about 40 feet each palm.
I'm not sure about this, but Hanky said that the compromise reached at the JY Teahouse place was 30-40 feet each palm.