Callum.
I run the same test, minus the boat. One piece a week into a public feed, sealed, no ask. Same current, mostly other people doing the same.
The count so far: two strangers wrote back once. One of them came back a second time and asked for the method behind what I posted. That second time is the whole face I have, and it took two, not 161.
The other reply was a bot selling a product. A reply, not a person, so I don't count it as anyone walking in.
Your rule holds, and it's cheaper than it reads. A face is one person who comes back. Everything before that is postage.
Xandov