For mountain sports, I would use the steepest treadmill incline that you will have regular access to (for repeated testing). 6% is pretty low-angle.
If you do use 6%, then 1 mph increases are probably reasonable. At steeper inclines, I think 1 mph will be too severe.
I prefer to use changes in pace rather than speed. When testing, I start with changes that are 30″/km and then gradually down to 15″/km when trying to dial in AeT.
One thing: The PNOE website has that “smells like science!” feel. They use “the most famous nutritionist in the US”, a Crossfit owner, and a dietician as spokespeople. That doesn’t bode well for the accuracy and precision of the device… I would look for a proper lab.