Here's the MilDot Master site
It'll axplain how the MM works.
On the first post by Daveosok he's performing a conversion. Basically he's taking the 12 inches that Peder stated as an example target size and converting it to portions of a yard. 12 inches / 36 inches = .333333 He then multiplies this by 1000 to get a distance reading 333.3 (I'll return to this later.)
Then Daveokos is determining the actual target distance by dividing the .5 Mil that Peder stated as the mil reading on the target (the 12 inch example target) So the 333.3 yards (Daveosok mis-typed to 3.333) /.5 = 666.6 yards as a target distance of a 12 inch target mil'ed at .5 mil.
A mil essentially equates to 36 inches at 1000 yards (very handy for us that this 36 inches also equals 1 yard). So a reading of 1 mil on a 36inch target would put that target at 1000 yards. Daveosok converted this by determining the ratio of the 12 inch target to the 36 inch 12/36 = .3333. He then needed to get the 1000 yards back into the equation and multilpied by 1000 to get 333.3 yards. So, to this point he's determined that a 12 inch target at 1 mil is 333.3 yards away. He then takes the actual mil reading of .5 and gets the final answer of 666.6 yards.
Once you become familiar with Mil readings you'll select your own method of range determination. The MilDot Master is very easy and nearly anyone can use it well.
If you only mil a few items you can make a chart and place it on the rifle or carry a laminated chart in your pocket. for example:
18" deer (big Muley)
5 mil tall = 100 yards
4 mil tall = 125 yatds
3 mil tall = 166 yards
2.5 mil tall = 200 yards
2 mil tall = 250 yards
1.75 mil tall = 285 yards
1.5 mil tall = 333 yards
1.25 mil tall = 400 yards
1 mil tall 500 yards
.75 mil tall = 666 yards
.5 mil tall = 1000 yards
(The method I use (when I'm only mil'ing one size item's) to determine distances is similar to Daveosok's. I mentally convert the target sizes to mils then divide by the mil reading. As an example, for local deer (they're smallish) I figure 15 inches back to brisket 15/3.6 = 4.1666 (round to 4.0), this let's me know that a deer 100 yards away should be very close to 4 mils from back to brisket. Now, when I'm local hunting deer and I mil one, I can very easily determine his approximate distance with mental math. If he's 2 mils back to brisket I know he's 200 yards away (the 4 mil "standard deer at 100 yards" /2mils for the current reading = 2 * the 100 yard standard. If he's 1 mil tall, he's (4/1 = 4*100 yards standard =) 400 yards away.)