In the measurement view, the group of lines where you're currently at is highlighted in the wing in red and magnified. By this, you can see exactly the values of your last readings, even if using a smartphone to run this application.
During the measurement, the line length differences are calculated relative to the first reading (indicated with ref).
After your first reading is made (usually A1/A1a or similar), the length might be e.g. 10mm shorter than the nominal value given by the manufacturer.
Every reading thereafter should also deviate by around -10mm from its respective nominal value to be considered “in trim”.
If your second reading is then 20mm shorter than its nominal value, the diff will show -10.
The third might be exactly as long as nominal and therefore diff is calculated to be +10.
By this we check, if the measured values are plausible and will give a warning tone and show the specific reading in black for your reference. With the back button the last value will be deleted and you can re-measure the specific line. In the wing view you additionally can see the colored highlighting for lines that might be off.
With the stop button you can interrupt your measurement session and continue later on. In order to continue you can just click the start button again. The first empty field will be selected and you can start over where you've left off.
If you would like to start your measurement all over again, you can delete all entered values with the reset button in the tool bar or reset a group of lines with reset readings button next to the respective main line.