Internationally, the preferred indoor range distance is 6m, though in some areas they prefer to add an additional 39inches to that.
Personally, all I have room for is 5 and 6M in the garage.
The ideal would be a 10m/33 ft distance if you have the room.
This way formal accuracy testing can be done, and it gives you the additional option of using the NRA 25 ft range also.
In the Garage, I use a 24"x24" L shaped aluimium diamomd plate Backstop a caboard filled box with sheets of cardboard inside. the rifles such as a fully charged Benjamin 392 will often have it's pellet contained inside the cardboard box, and if it does go through, safellyshatter a pellet against that backstop plate.
Outdoors on my 10m range I use a similar backstop with1/18" plywood sides (to contain a side richoet). I have lined my backyard fence behind that, with a fullsheet of 1 1/8" T&G plywood, just in case of a stray...
Urban "Home" shooting is self regulated to the guns power, discharge note, and good common sense!
HTH,
ZVP