I need a spotting scope primarily for the range. Shooting a .30 on paper targets at 200-300 yd. What X is good for that?

22x has worked for me for the last 40 yrs. or so.

My 15-45 will do the job up to 300 yards and is ok but I would rather have better optics at a lower power with a clearer view then zoom in and try to see with heat waves or the mirage effect...

I would agree that a spotting scope with a 15-45X range should be more than sufficient for a typical rifle range type setup.