I use an 8.5-25X and it seldom gets below 20 even with the mirage acting up.
Expert benchrest and F-class shooters often use higher powers. I don't know what the high end for a sling gun is but you would not be hurt with a 12-42X.
Actually I don't know what use the low end of a 4:1 zoom ratio is to a target shooter. Something like a 15-40 would be good. The very expensive March 35-50 is a real expert's glass.