Not only does the application seem promising, but what I did was that I took several tools to measure the apps ruler and, after running some tests, concluded that the imperial system is just right. I have taken another tool (my only tool—12 inches—which can also measure in centimeters) to measure the in-app ruler in centimeters, and, having observed my tests, I can confirm that the metric system is also accurate.
One of my only two complaints however is that, for every half of a half of an inch, the marker is about the same length as one-quarter-markers and three-quarter-markers (both of which are for every half an inch), and my suggestion for them is that they be slightly lengthened but obviously still shorter than half-inch-markers. My second and last complaint is that the free ruler is only 8 5/16 inches long, which for me is a little weird because a popular length for imperial rulers is a dozen inches (a.k.a. one foot). I have not seen a ruler with such a length, so, yeah, it is just weird and unusual.
My conclusion: with just minor flaws, this app is great, and it works better when we have a sheet of paper or some other material so that we could mark our points.