This target is 100mm in diameter (3.93 inches) which correlates to 1.0 Mil-Radian @ 100 metres. 

So... running a Mil-Rad scope / reticle? This target size is perfectly for practising your Mil-ranging.

Don't forget the formula:

(Target Height in Metres x 1000) / Height in Mils = Range to Target in Metres.

Or:

Target Height in Millimeters / Height in Mils = Range to Target in Metres

Precision laser cut from 12mm thick Australian made Bisalloy 500 steel.