No doubting you but need theory here. Why 3&1? Why wouldn't this be previous spot like roughing the kicker? Obviously not PSK because it happens at the line.
Roughing the kicker can only happen during a kick
play, so the Basic Spot - and, ultimately, the enforcement spot - for roughing the kicker is, indeed, the Previous Spot.
For other personal fouls, the Previous Spot
could be the enforcement spot, but it might be some other spot.
4/10, 50. A is in a standard punt formation, with A11 some 13 yards deep, clearly a SKF. Immediately after A55 snaps the ball, B99 blocks A55 to the ground (foul). A11 receives the snap and throws a legal forward pass to A88 who catches the pass and advances and runs OB at the be B-39. The Basic Spot for this personal foul is the Previous Spot, because it occurred during a pass
play. Since it is a foul by B, application of the 3 & 1 principle at the Basic Spot yields enforcement of the penalty from the Basic Spot, 15 yards, first down. A, 1/10, B-35.
4/10, 50. A is in a standard punt formation, with A11 some 13 yards deep, clearly a SKF. Immediately after A55 snaps the ball, B99 blocks A55 to the ground (foul). A11 receives the snap, advances and runs OB at the B-35. The Basic Spot for this personal foul is the end of the related run, because it occurred during a running
play. The related run is A11's run, which ended at the B-35, so the Basic Spot is the B-35. Since it is a foul by B, application of the 3 & 1 principle at the Basic Spot yields enforcement of the penalty from the Basic Spot, 15 yards, first down. A, 1/10, B-20.