All you NEED to shoot good groups at 300 is good iron sites.
Magnification is not needed, it is a preference. Your choice may change with experience. If you are only target shooting, the scope you mention will work. What brand is it? Quality glass at lower magnification will be clearer than cheap glass at higher.
If you might hunt with it you should keep your lower end near 4X for close up shots.
You can't fix stupid....however ignorance can be cured through education!