Re: Just want to make sure I understand "mil"
Actually, it can be done with a SFP scope, as long as the turrets and reticle match. Otherwise, you have to do a conversion. For a SFP scope, the manufacturer will tell you at which magnification the reticle is calibrated. Usually it's the highest mag, but not always. Depends on the scope.
Obviously, it's easiest with FFP. No matter what mag you're on, the reticle is calibrated as it changes in apparent size proportionally in accordance with magnification, so no thought is required.
With a SFP scope, let's say a Nightforce 5.5-22x56 for example, the reticle is calibrated at 22X mag. If you dial the mag down to 11X, an object seen through the scope appears to be half the size relative to the reticle it did at 22X, because the reticle does not change size as the mag is dialed down. That means each hashmark on the reticle now subtends twice the arc length it did at 22X. In other words, the reticle didn't change size, but the object shrunk in apparent size by half, so each hashmark subtends twice the arc length relative to the object as they did at 22X. Therefore, the estimated size of an object (or estimated distance between POA and POI) made using the reticle at 11X is only half of the true value. So you would need to double any hold or adjustment made using the reticle with the scope on 11X.
The major caveat to using this approach with a SFP scope is that the eyepiece may not have an "11X" mark on the mag dial. Even of it does have an 11X mark, the actual magnification may be 11.4X or 10.7X when you actually have the eyepiece lined up to that mark. So you need to know exactly what mag a specific mark represents, which can be done at the range using a calibrated grid at an exact known distance. Some people will go to this length to specifically characterize their equipment, however, most won't.
Depending on what you're doing, it may be that this approach with a SFP scope, even if it's not dead on, will be close enough to allow the correct hold or adjustment. For example, you're using your NF 5.5-22X at 100 yd set on what you think is "11X", and you see that your impact was exactly 0.5 mil low by the reticle hashmarks. This would mean that you'd have read the POI as being <span style="text-decoration: underline">1.0 mil low</span> <span style="font-style: italic">if the scope had been set on 22X</span>. In this example, chances are pretty good that making a correction based on your 1.0 mil low estimate (0.5 mil by the reticle hashmark x 2) would put you pretty close at 100 yd. However, the longer the range, the more you might be off. In general, although you <span style="font-style: italic">can</span> do it with a SFP scope, it's not ideal. Plus, it's one more thing you have to think about if you're not using FFP.