Or, use the Measure app on your iPhone to get the slope in degrees, then convert using a website converter like this one
Angle Degrees to Percent of Slope Conversion Calculator - CalcuNation.com unless you can remember how to use your slide rule to do trig functions.
Using the ruler was a lot easier and quicker when I was doing it everyday, By the time I retired, flip phones were state of the art.
I had slope gauges on my dozers and graders that you could look at anytime and know exactly where you were at.
Really good operators said their slope gauge was built into the seat of their pants and I was always amazed at how close they could hold it. I also had 4 to 10 ft slope boards that were adjustable for measuring road and shoulder slopes most of which were 1/4 to 3/4" per ft. The first cross slope gauge for my grader table I made was a graduated board with a straight piece of coat hanger dangling over it, Just like the Romans used as a level or slope gauge, but I'm not quite that old!!!