However, if you have a more complex roof, you may need to calculate pitch section-by-section. Exclude the gear's teeth from your measurement. When the bubble says it is level, then have a second person measure the distance from the end hanging in the air straight down to the roof. You can use the Pythagorean theorem which is used to calculate roof pitch multipliers to calculate the run of your sloped roof.
The roof pitch calculator finds the length of the rafter and the roof slope in degrees and in percents. To measure the gable run, you need to look at your roof from the side and picture it as a triangle, with one unknown refer to the roof diagram above. Don't feed them and let them suffer. A smaller pitch corresponds with larger teeth, and smaller teeth belong on a gear with a larger pitch. Measure the radius of the gear.