Range estimation is only one advantage of the mildot scale. They are also very effective for holdoffs for wind or a moving target. One mil is measured from the center of one dot to another, each dot being .75 of a mil. To estimate the hold off for wind, or a moving target each mil equals roughly 3.5 MOA.
Laser rangefinders are cheap enough now, that the serious long range hunter should have one. They are faster and more accurate. The mildot scale work pretty well for range estimation if you know the precise size of your target or the size of another object very close to it. Small mistakes in the size of the target can mean enough of a mistake in the range estimation that you will miss, and the longer the range the greater the error. If you are determined to learn to use the mildot scale for range estimation, the laser can be used to check your calculation.
As an example lets say that you estimate your target size at 12" in height, but in reality its 15" high. The first example we'll say you measure the target height at 2 mils and in the second at .3 mils.
12 X 27.778 = 333/2 = 167 yards
15 X 27.778 = 416/2 = 208 yards
12 X 27.778 = 333/.3 = 1110 yards
15 X 27.778 = 416/.3 = 1387 yards