From PANS-ATM (ICAO doc 4444):
a no-transgression zone (NTZ) of at least 610 (2000 ft) wide is established equidistant between extended runway centre lines and is depicted on the radar display
Furthermore it states that the distance between the runway centre lines shall
not be less than 1035 meters
subject to requirements on the surveillance equipment. The worse the surveillance performance, the larger the minimum distance between the centre lines.
I guess the math is correct but you forgot to add some margin for the (in)accuracy of the radar.
ATCast