The hand should be parallel to the strings and not rotated (there's a tendency, sometimes to put the hand in a 30 degree angle, that should be avoided in that kind of pull off with those fingers.
The fourth finger should not be overextended.
All joints(when possible) should be "bent" in a way that makes the tipjoint phalanx similar to a "nail". When you hammer a nail, you make sure the nail is 90 degrees to the surface.
The pull off should not be done with too much strength (it's not a "fortissimo pull off" to make a metaphor). Also, there is a tendency, somtimes, that when one plays in a strong volume with right hand, the left hand fingers (when performing a pull off) may also "contract" that strength. Best case scenario is to dissociate both hands in terms of strength. The right hand may play in forte (with strength), but left hand should be soft when performing a pull off. You may wanna try playing those pull offs in "piano" dynamic, at least to test the strength you're using for pull offs.
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