Half the distance to the goal--When?

[trainman52]

Please excuse me for my newbe question, but I have not spent much time looking into the actuall enforcement of penalties untill just now. I am not finding anything that tells you when you apply half the distance to the goal. I am going to assume that it is once you can no longer apply the full yardage of a penalty? i.e. Illegal personal contact (0n B) at B's 10 yardline. at this point it would have to be placed at the 5? Thanks!