[jLoy88]

Section 2 Location of Foul
HALF DISTANCE PENALTY
Article 1 If a distance penalty, enforced from a specific spot between the goal lines would place the ball
more than half the distance to the offender’s goal line, the penalty shall be half the distance from that spot
to their goal line.

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15 yard penalty on the 30 yard line = 15 yard line.
15 yard penalty on the 26 yard line = 13 yard line.

If the result of the penalty would otherwise be greater than half the distance between the line of scrimmage and the goal line, than half the distance is enforced. So for measurements;
5 yard penalties - can only be 1/2 distance within the 10 yard line
10 yard penalties - can only be 1/2 distance within the 20 yard line
15 yard penalties - can only be 1/2 distance within 30 yard line.

this applies inversely for moving forward as well as backwards on a penalty.