The Contour has a reputation of very good accuracy, so you would multiply the reading of 80 by 1.2 to increase the reading by 20%, getting 96 as 20% greater.
I just measured the Dario dongle to see the case requirements. The opening in the case needs to be at least 10mm by 15mm if the case is more than 5mm thick. If the case is thinner, no cutting should be necessary.
