You can range at three magnification settings. 20x is obvious, 6.5x and 10x are also easy. 10x is going to be extremely useful if you're shooting and its hot with a ton of mirage when 20x gives you nothing but distortion.
To find 10x (half magnification) measure anything at 100 yards (3Mils for example) and turn the magnification down until it measures half of what it just did (1.5Mils to continue the example).
Now that you know where 10x and 6.5x are, here's the simple solution: Mil the target just like normal, if you're at 6.5x just take the mil reading without worrying about the difference yet. Once you get your yardage from the Mil Ranging Formula, divide it by 3. For 10x divide it by 2.
Example: 40" E-type vertical target multiplied by 27.78 (Constant for answer in yards) is 1111.2, divide that by a measured (at 6.5x) 0.5mils gives 2222.4 yards. Divide by 3 to account for 6.5x and with one quick additional step 6.5x magnification gives an accurate range estimate of 740.8 yards.
Simply run the damn formula like normal, divide by 2 (half power) or 3 (1/3 Power) and poof! SFP scope can range at more than full power. And as an additional benefit to guys like me who don't do tactical competitions where FFP is a very real advantage, I get the fine reticles I love.
Hope that helps your ranging OP. Take your SFP scope and run, FFP isn't the only way to go and there are still a few of us who don't like them for our own uses.