As if you did not know
… the ideal target for a solid whack is the bottom of the cheeks at the point where the hamstrings meet the ass. The so-called “sit spot.” It stings like the dickens at this spot, and makes for a memorable spanking. Of course, the rest of the bottom should not be ignored, but stay below the top of the ass crack. Anywhere in that zone is good, but the highlighted spot is most effective.
Via Strict Julie Spanks! – original source unknown