I also failed to mention that the exit pupil of the intended scope would likely be a problem in low light conditions. Anything above an 8 X with a 40 mm bell may be problematic (that puts your EP AT 5 which is about all a normal eye can use. Below 5 and that's when potential issues arise. At 18 X you're at 2.2 EP). You also have parallax concerns once you get over about 10 X. Again, I'd say 9-10 X at the top range is about perfect...assuming you have decent glass.