The trick is in geometry as stated:
At 200' AGL on a typical 3 degree path distance to travel is 3821', which converts into 1165 meters. As lights are usually easier to determine than their surroundings a factor of 2 is applied to take into account their effect, reducing therefore the RVR minimum to 582m.
As angle may not be exactly 3 degrees and MAHt may not exactly be 200' AGL, changes to this figure are made using simple trigonometry for creating usually higher RVR values when higher decision heights are used.