Tom, I'm not sure that "effective range" is used or meaningful in connection with optical instruments. E.g., if visibility were 25 miles, anything sighted would be magnified (for example) thirty times. I suppose that could be a thirty times larger blur ;-)|
My impression is that the signal OP on Sugar Loaf, for example, afforded a wide area of coverage--a deep penetration of Northern Virginia, the Valley, and surrounding areas. A practiced eye would have looked for smoke, dust, fire, and other indications of troop movement and activity, not just "pin-point" identification. I'd suggest leaving it at "a magnification of approximately thirty times," or something to that effect.