Knox. Folks measure differently and get different results, but you can use either one. Doesn't matter as long as you know which one you use and use it consistently.
On my square, the same model Bisch is using in his picture, pushing the back of the frame legs/arms, which extend 1/8-inch further than the center of the square, all the way to the string gives the same result as a ruler or tape measure. Most people do it this way.
If you stop at the first notch, the measurement will be about 1/8-inch more than that. If you push the feet tight to the string, the measurement will be right on, i.e. the same as a straight edge ruler measuring from the throat of the grip to the string.
If you measure like Bisch and some others, placing the indented part of the square on the riser, it will give a measurement 1/8-inch more than a ruler or tape.
Doesn't really make any difference as long as you do it the same way each time.