OK, I have just been out with a tape measure
, and can confirm that the majority of poo falls between 5 and 6 inches from the edge of the perch. Having seen how they perch, I would allow 9 or 10 inches between the edge of the perch and the wall, for optimum tail-feather clearance
.
One of the best things we did was to place a sheet of coarse metal mesh underneath the perch. I've modified it a bit since
this photo was taken, but the principle is the same - basically the poop all falls through the grid and into a plastic tray. They can't therefore scratch it about, which means everything stays very clean, and all you have to do is throw a handful or two of the old hemcore bedding over the poop every couple of days to absorb the moisture, and then empty the tray into the compost every few weeks.
HTH!