How is the % incline measured or calculated?
number of meters of elevation over 100 meters.
to find the angle take the arctangent of the %.
to find the % take the tangent of the angle.
I always find it somewhat weird that a 45deg incline is 100% grade. That just seems not right. 100% grade seems like it should be vertical, but that would be an infinite grade… One of geometries little quirks…
“Rise over run” (ie hill rises say 200m and is 4km long (in plan as measured on a map for example)) If you use the distance from your trip computer, that would be measuring the hypotenuse of the triangle that the hill represents, and your “run” distance would be artificially long, meaning that your gradient answer would be slightly less than reality.
The above example would be an average 5% gradient and you should be in your big chainring! 
The small chainring is reserved for over 7% or if you’ve popped and are struggling home like a little girl. Or if you are fat and weak like TriDork.