Range (distance) will determine the need for parallax adjustment, not magnification. Also, that non-adjustable Minox should make the rifle shoot most-accurately at that distance, and worse at others (if indeed it was set for 100 at the factory as they may have screwed up; or it could have intentionally been set differently, most likely higher on that scope if it was).
Being a firm believer that variable-power scopes are overrated (and overpriced), I have a Sightron SIII single-20X scope with a handy "1/2 Mil" reticle that lets one get a little tighter when using just reticle holdover. It has parallax adjustment located on the eyepiece like the SWFA "Supersnipers" did. Too bad has MOA clicks and doesn't have glass as good as the other S3s; otherwise it would be my favorite scope.