class 1 & 2 testing bay

Having recently been approved & undergone relevant training,I now need to mark out the standing area for headlamp aim testing,directions are vague to say the least,I wondered how far from the rail mounted headlamp aim tester the datum lines have to be & also do I have to mark a centre line @ right angles to the datum lines?do I also have to mark out the complete standing area of 2.5 x 2.0 meters?Any help or views greatly appreciated