Call Distance - Boobook Owl

["John Tudor" jtudor2005]

There is a Southern Boobook calling somewhere near my house in the
Adelaide Hills. I can hear it clearly (although faintly).

In order to narrow down the search area, can anyone indicate what sort
of distance over flat, fairly open farm land, that owl calls can be
heard with just the ears (no amplification) I'm guessing it's within
about 500 metres of home.

Is there any rule of thumb based on  bird size??

Regards
John