If it is a SFP optic, then the subtensions are generally "calibrated" for ranging objects at the max power setting as you noted. You CAN use any of the power setting to attempt to range an object, but you will always be more accurate at the correct setting, especially as the distances get longer.
If the goal is to provide a precision shot under field conditions, then you need to determine the level of accuracy and precision that is acceptable and go from there. For example a .1 mil discrepancy won't matter much if your target is 300 yards away and your goal is a shot on the torso. But the same discrepancy at 1500 yards will matter a great deal.
If the goal is simply to provide a rough estimate or distance to an object, not being a target, then you can use your idea, but again you will always be more accurate using the "correct" method.