How does the subwoofer distance setting work?

Well since my post got removed from r/home theater, maybe someone here can help. I have a Yamaha RX-V385 and calibrate it with the YPAO mic. But when I repeat it several times, the subwoofer distance varies from 7.8 to 14.8 feet. Usually it's around 12 feet so I went with that setting. But why does it vary so much and how can I make sure I have the right setting? The actual distance is 6.8 feet and I read that it's usually a couple feet more but this seems excessive and its a wide range for what it measures.

Is there a way I can play a test tone or something while I play with the distance to hear what setting is best? Or should I just go with 12-ish feet?