That formular is wrong. The correct formular is (according to the official TÜV theory test questions):

2x(v/10)x3 (well technically they only calculate it for 1 second so I added the 2 infront).

That is, at the same time, slightly more than half your speed meters.

Your formula looks like the formular for the “Bremsweg” with a “2x” added infront but thats not how it works because time is not a variable there…