Every measurement setup is a bit different. If you did not yet setup the offset in your profile settings, you can do it prior to starting your measurement below the section where you entered your loops.
The offset does not affect imported values or measurements that are in simulation or finished. It only there to add/subtract the given value from the reading of a connected laser.
We furthermore provide the option that, via a settings button in the toolbar on the measurement page (the page after you finished entering your loops, see next chapter), you can set the offset value to make the measured lengths better comparable to the nominal values.
The nominal line length is given from the carabiner all the way up to the lower surface of your wing. Depending on the manufacturer the fabric loops where the lines are attached, are also included. Check the manual for more information.