excellent advice already posted - so just adding my .02 cents. Let's think about what milling the target is and isn't. For reference, I am talking about being in a steady position, with the rifle bagged on the rear or toe of the stock and on a bipod, using a mil reticle optic on the rifle to see what the target subtends in mil's (how many mils, in whole or part covers the target, either in height or width).
we need to know the mil ranging formula(s), the targets size, have a way to do math or a cheat sheet for it, and to be practiced enough in reading the what we see to be within .1 mils accurate.
Then taking the target measurement in feet, yards or inches or the metric measurements, doing the Mil relation formula and determining the range to the target in either yards or meters.
so what mil-ing a target is is a redundant method, back up to a laser etc., when time allows, to achieve data book info and range cards. What it is not, is a quick method to engage a target. shit the target will have moved in a dynamic real world situation by the time all this is done.
so, what comes in is 'flash milling' or getting a quick down and dirty mil reading at fairly close range 100 to 600 yards or even beyond (telling me i need to get lower or figure out a different method to use) knowing for instance that most humans are 39" from top of head to crotch and that equals 'x' mils at 'x' yards, and then holding the correct adjustment and sending the round downrange.
This 'flash milling' you can do from positions other than prone, say sitting or standing because typically it is on a target close enough to accomplish a flash mil reading on, ie., 600 yards and in
lasers work faster for dead nuts accuracy, flash milling works wonderfully too, and combined with a flat trajectory caliber like 7mm mag, 300 mag, etc., you are stacking the odds of hitting the target even more in your favor
just my thoughts -
probably tons of resources out there on flash milling, namely the SOTIC sniper manual (latest version) which is like 2010 or so.
side bar to this topic is that from the muzzle to 600 yards or so, you should have your hold-overs memorized and/or written on your rifle. so again you can quickly engage a pop up target by holding over or under. on a human sized target like a IPSC or fbi or steel this gives you 37 inches of target to hit, and a hit in there somewhere is good - assuming you are shooting a caliber that has terminal velocity performance and not just steel plinging performance. horsepower downrange = success
we can call this the 'martial side' or warrior side of precision shooting, not the hit a fucking dime at 100 yards side. so what right? well, if we are carrying our gear then entire time, then that's what . . . not pulling endless gear out of a vehicle or having a range day (all good times but different than running light and lean).
What does it take to hit a target? If you gave yourself 20 rounds and 10 targets from 100 to 750 yards, and that is it . . . no Kestrel, no calculator, no rangefinder, nothing else, just the rifle and ammo, what is your expectation of hitting those ten targets with each of the 20 rounds given? down dirty, brilliance in the basics, essence, budo, zen, get some!
if time allows you can do lot's of cool ass math and layout triangles and knowing the sides figure the base, or vice versa - lot's of resources on this too.
bottom line don't get fooled into thinking mil-ling is the 'way' to engage targets - cause it ain't. it is a method to measure range and a super excellent, legacy way of measuring range, we should all know. measuring takes time.