With the Leupold mil-dot scopes (mine's the 3.5x10x50) the mil-dots are regulated for the highest power setting (10x on mine). At that setting the distance between dots, from center to center, is 3.6 MOA (3.6 inches).
For the non-calculator inclined, like me, that's...
3.6" at 100 yards
7.2" at 200 yards
10.8" at 300 yards
14.4" at 400 yards
etc, etc, etc
36" at 1000 yards
You must know some dimension of your intended target to use the dots for range estimation. A deer is 18" from back to belly so, if that fits between two dots (exactly), it's 500 yards away. Three dots = 250 yards etc. I find it much easier to use a compact laser rangefinder but knowing how to do it the old fashioned way is a good skill to develop.
Perhaps the best use by a hunter type shooter is to sight in as usual and just use the dots as "hold-over" points. For instance, my PSS is sighted dead-on at 100 yards with all knobs set to zero. I know it's 6 clicks (1.5 MOA) up for 200 yards. In a pinch I can just hold over 1/2 the distance to the first dot. As with any range finding reticle, or scope/gun combo, you just have to go shoot it enough to know how it works for you. NEVER trust data tables until you confirm them at the range with your gun and your ammo.
Mikey