i've long contended mil ranging isn't practical. it used to be very common part of sniper matches and everyone sucked at it. everyone.
in the past several years, it's really only useful in two situations:
1. to get you in the ballpark so you or preferably your spotter can give you a correction and hit a target on the second attempt
2. when your laser is giving you multiple distances and you have the time to mil the target to see which distance is right. this is fairly common when you have trees or cars or something that appear on either side of the target but that are actually much closer or farther away, or if your target is on top of a hill with another behind it, etc.
with silencerco radius selling for $500 and lots of other decent range finders on the market, the only people mil ranging anymore are doing so out of nostalgia
that said, if you want to be semi-decent at it, here are a few tips
1. measure the largest object you can. your error will be much less significant that way. e.g. don't range the 6" plate, range the 48" piece of rebar it's hanging from
2. practice ranging different shapes (circles, diamonds, squares, etc) and objects of different colors. you may be surprised that you get different measurements on different colored objects
3. never moon a werewolf.
4. poorly designed reticles often have the smallest graduations at the extreme edges of the glass, where distortion is the worst. it may be worth changing magnification or using a part of the reticle near the center. it's not just your glass, but also your vision, if you are for instance, straining to view the top of the FOV
5. get a mildot master and make a mil cheat sheet like the one pictured below. aside from being inaccurate, mil ranging takes forever and you'll miss a chance to pull the trigger if your target wanders off while you're jacking around with a calculator.
6. you basically have to build two stable positions when miling; one to measure the target, then you go to your mildot master or cheat sheet and dope card and then build another position to take the shot. practice not sucking at that. put your dope where you can get it without moving your head off the stock if possible. (e.g. i used a retractable lanyard)