A mil scale reticle is not metric - it has no measurement associated with it. A radian is a trig function. If you measure from the center of a circle to the edge that is a radius. If you take that distance and measure along the edge of the circle to the next point and then draw a line to the center of the circle you make a pie wedge shaped section - that is a radian.
A circle has 2 Pi radians or Pi x 2 = 6.283 (approximately). There are 6.283 radians in a circle. A "mil" is a measurement of 1/1000 so if you multiply 6.283 x 1000 = 6,283 mili-radians in a circle.
For minutes of angle, there are 360 degrees and 60 minutes or 21,600 minutes of angle in a circle.
To convert between them - 21,600 / 6,283 = 3.44 MOA per mili-radian.
So far - no fixed measurement of inches, feet, centimeters, miles, kilometers, etc. Each is just a description of the arc of a circle.
Now, if you want to use a scope with a mili-radian reticle for target range estimation you can do that IF the scope has a front focal plane (FFP) reticle. You need that because the reticle mil measurement needs to stay proportional to the amount of magnification.
One Mil subtends 3.6 inches at 100 yards, 7.2 inches at 200 yards, 14.4 inches at 400 yards, 28.8 inches at 800 yards, and a handy 36-inches (1 yard) at 1,000 yards.
So, you can use the FFP Mil reticle to estimate distances. The calculation is: (height or width of the target in yards / number of Mils on the reticle) x 1,000 = yards.
If you have a known target height - let's say 6-feet that's 2 yards. If the target takes up 4 Mils on the vertical reticle then (2/4 =.5) x1000=500 - so your target would be 500 yards away.
As you can see - all you have to do is understand the system whether it is Mil or MOA, and apply it to your own use.
I like FFP Mil reticles as they are simple to use for range estimation once you understand the system.