Measurement on a handheld device has a slightly different interface than measurement on desktop (pc/mac). See demo and documentation for pc here: 3D measurement
The rationale for having a slightly different interface for the measuring tool on the handheld devices is simply that big fingers on a small screen is not as accurate as the cursor on a big screen. We have therefore made a fixed measuring cross in the middle of the screen and you navigate your 3D so its fixed on or between the objects you want to measure. This ensures better accuracy of the measuring point.
Video recorded on Mobile, Samsung Galaxy S8