Or maybe even 2.7NM vice 3NM
It gets more complicated than that.
Upon verification Aircraft A tells me he is at 8,000 ft, radar says 7,800 - within tolerance. Aircraft B says 7,000, radar says 7,200 - within tolerance. The tracks cross and I have 600 ft between them on the screen, but I am using a 1,000 ft standard!
Agree with topzalp.
We also have a dumb idea here of applying a tolerance on methods used to measure the distance. If we use a Bearing Range Line measuring tool we sometime have to add a mile or two, but for comparing with the scale marker (a fictional concept denoting "eyeballing" the standard) no tolerance is required.
I once saw one approach controller stood down coz he had 5nm by eyeball (wake turb sep) and the tower had a BRL on and whined when it clicked down to 4.9. Seemed a bit silly.