[Mike O]

Chris,
Lowpoint can vary depending on the swing. On a practical perspective you'll probably find it close to opposite the left shoulder. Although you could hit some balls and find the lowest point of the clubhead path for your swing and it may be somewhat different.

If you were to perform a putt, chip or other swing where the arms, hands and club were "frozen" and you just rotated your spine - then the lowpoint is opposite the one moving center - your spine.

In the golf swing you have multiple moving centers, so depending on the sequence and timing of each center's (shoulder, wrist, etc) lowest point the actually lowpoint of the clubhead will/could vary slightly from player to player, or swing to swing.