@zooker
I'm not an ATCO (hopefully in two weeks I'm officially a tATCO though) but I agree with you. The only reasonable usage I've found is approach/departure. If you do 180 knots you do 3 miles per minute and the approach speed is less. Say it's 150 knots which means 2.5 miles per minute, so at 6 miles you should in theory have at least 2 minutes to work with for departures.
Apart from that, I have no clue why the insist on it. But I guess being good with maths might indicate some aptitude they want to have in their atcos.
@mts
I'll get back to you later with a solution.