With an FFP scope you can readily range at any power, as the size of the target changes at the same rate as the reticle when adjusting the power.
This is not true if the SFP scope; the reticle remains the same size regardless of the magnification.
On the SFP, if you take the power you want to range with, and divide it by the power you're supposed to range with, you'll end up with a number. Take that number and multiply it by the MOA/mil number you got when ranging.
Example:
You have a 6x-24x scope that is set to range at 14x from the factory, and you want to range on 24x, here's what ya do:
Range as you normally would. Take the mil reading and divide it by 1.71. Then factor the new mil reading into the standard formula.
1.71 is obtained by dividing 24x by 14x: 24 / 14 = 1.71. You divide the power you want to range with by the pre-set ranging power.
You can use this method for any power on your scope, i.e., if you want to range on 20x: 20 / 14 = 1.43
So, you have a known target size of 20". You range and get a reading of 2.225 mils. Your formula when ranging at 14x would be:
(Size target in inches X 27.778) / mils = distance in yards
(20 x 27.778) / 2.225 = 250 yards
If you were ranging at 24x, take your mil reading of 2.225 and divide by 1.71:
2.225 / 1.71 = 1.3 is your new mil reading.
The formula would be: (20 x 27.778) / 1.3 = 427yds
Easy thing to do is to go ahead and figure out the adjustments ahead of time and write'em down.