You can still do ranging at other powers, but the subtension of the reticle is changing. So my PST is MOA subtensions at 24x for measuring and ranging, so at 18x each subtension is 1.25 MOA, 2 MOA at 12x, and 4 MOA at 6x.
Should be the same for your MIL reticle, if your subtensions are 1:1 at 18x, they will be 2:1 at 9x, and 3:1 at 6x.
So all you really need to keep in mind is how much subtension you're measuring at whatever power you're using and then use the same formula you've always used.
IE, if you're using 9x and the target measures 3 MIL by the reticle, it's 6 MIL actual measurement, if you're using 6x, it's a 9 MIL target (by the reticle).
The long and short is that the ranging calculation won't change at all, but the target measurement method will. Fortunately, the math for that is really easy.