Yes, what you asked, and how JimNM described it is how 2nd focal plane scopes work. I had a FFP scope but since I really like to shoot on 4-6x, it was hard finding the target quick; but they're great for ranging & BR work.
As you stated, your mils are true at 10X. My Hawke 4x12 Air Max has its dots true at 10X. Some companies have them try at max magnification. I think BSA scopes are true at 14X. It all varies and here's how to be sure.
Counting on you having an accurate laser range finder or good old 25' tape measure you'll have to move 12 x's to an exact 100 yard mark. Set up a 36" yard stick, or cut a block of wood at exactly 3.6 inches. Paint it so your reticle stands out against it. Mils are measured from C-T-C. center of dot, to center of dot, divided by 10 parts 0.1 mil each. Thus a mil/mil scope moves your POI one tenth of a mil per click at 100 yards.
Once you have your 36" (3 feet) yard stick & 3.6" wood block at 100 yards, get settled in on a sand bag or bipod. I suggest complete stillness, control. You can also make a bigger block of wood, or sheet of cardboard, whatever. The point is to have it marked out straight up & down at 3.6" intervals. Play with your zoom until it matches exactly. Reticle in center of dot to center of dot. This will tell you how true, and where your scope is true. Once you know that, and can either range find or if a good estimator of distance, make use of landmarks, you can start dividing or multiplying your hold over/under by distances & magnification.
The mil metric system can be used just like ours. It's quicker though to estimate tenths than it is to break down smaller SAE units beyond quarters. at least in my head. If your scope is set up right, you'll find where it's TRUE at 100 yards at what magnification. Putting the 36" yard stick should equal 10 mils exactly at some point on your AO. You may need to make your own scope tapes. Compare this later with what your eyes tell you is the sharpest focus achieved. Compare range marks on AO bell to range finder. Scopes usually do this best at their highest power magnification.
