Section 188.8.131.52 of the Guidelines for Tactile Graphics does state that an inch should be left on both sides of a line or all sides of an object to be measured to allow for the placement of an actual braille ruler.
The suggestion to leave the space above the ruler was due to the varying types of rulers. The ruler I have at my desk has inches on one side and centimeters /millimeters on the other side. If the student needed to measure a line in inches, they would be able to put the inch side of the ruler right under the line being measured. If the student needed to measure in centimeters, they would turn the ruler over and place the centimeter side under the line to be measured. However, if the student needed to measure in millimeters, because the millimeter measurement is on the bottom of the ruler, the student would have to put it on the top of the line in order to make the measurement.
I think the 1 inch space requirement all around a figure to be measured, was written so that no matter what type of ruler a student has or needs to use, they would be able to fit the ruler up next to the line to be measured without having to place the ruler on top of braille.
Good question! Diane