A Mil is a Milliradian, which means it subtends one yard of the circumference of a 1000 yard radius circle, or one metre of a circle of 1000m radius. More practical is to remember a Mil equals 3.6" at 100yds or 10cm at 100m, and increases by 3.6"/10cm for every 100yds/100m of range.
The idea of the Mildot system is that it acts as a sort of ruler. You look at an object of known size, compare it to the Mildot scale, and then use this formulae to find the range. Each Mil dot on a scope reticle is ¼ mil, the distance between the centres of the dots is 1 mil and the distance between the edges of the dots is ¾ mil.
Range (metres or yards) = [Target size (metres or yards) x 1000] / Size in Mils
OR
Range (in 1000s of metres or yards) = [Target size (metres or yards)] / Size in Mils
If a man is assumed to be 6ft (2yds) tall, and appears 5 mils high, then 2000/5 = 400yds. Distance from belt buckle to crown can be assumed to be half his height, so for the previous example 1000/2.5 = 400yds. When dealing with a 0.5mil value it is probably easier to double both figures i.e. treat 2000/2.5 as 4000/5. Likewise quadriple 0.25 and 0.75 values. 1000/3.75 = 4000/15 = 266.7 yards.
Not a complicated formulae, but also not one to try and do in your head in the field when under stress. Also, six footers are not average, even in Western Society. Average male height is around 5' 8" in socks, and female 5' 3". 5' 9" is about 1.9yds, 5' 6" about 1.8yds and 5' 3" 1.75yds. This makes the maths a little harder to do in your head.
You may not agree with the features I've selected, or the dimensions may be off for the targets you are likely to encounter. The average height of someone in Asia probably is quite a bit less than 5'9". To allow for this the cheat sheet is an Excel document with embedded formulae. All you have to do is decide the size of the object you want to use as a reference, and divide this by the number of inches or cms per mil at 100yrds, or 100m, depending on which units you are using. These figures are found in cells B2 and B3.
For example:-
11" long rabbit. 11/3.6 =3.06mil at 100yds
75cm high target 75/10 = 7.5mil at 100m
Once you have this mil value, enter it into B10-16 on either the Yards or Metres sheet (make sure you have not calculated the size at 100yds if you are using the Metres sheet). When you enter this value the sheet will calculate the rest of the table automatically. B15 and B16 have been left blank so you can add your own values.
It may also be possible to create a nomograph so that you can place a straight edge between the column for the object actual size and that for the size in mils and read off the range. However, the on-line nomograph drawing programs I have tried have not given me anything comprehensible.
There is another way to use the Millirad system. Hold your arm out straight, elbow locked, palm up with one or more fingers raised. Look at the finger with one eye closed. The width of your first finger is approximately 30 mils, the first and second fingers together 70 mils, three fingers 100 mils and four fingers 125 mils. This method is most useful for large objects such as vehicles. A tank can be assumed to be 10 feet wide and 20 feet long. 30 mils is equivalent to 10ft at 100 metres.
