Heres the problem with measuring turn radius, especially on a dirt track, the radius, on most tracks, change throughout the corner so there is never a "true" radius. Example, one of the best known tracks in the country, Carnesville, turn 3 is fairly gradual entry but coming through 4 it tightens up the radius a lot coming off, you have to take that into account.
Also most tracks you are very rarely ever in a straight line down the straights, which also effects things.
Type of kart and the way it was designed also plays a big role in stagger. At the biggest races in the country, the winners all may have differing staggers from 1 manufactured kart to another.
Find which stagger works the best for you at any particular track to make the most corner speed and you will be at the front.