That "pocket gauge" is an inclinometer. It's what we use, I don't have any issues with using the smartphone apps but I've seen them lie to me and paid the price after I already set a pole.
When I installed my parent's dish down in Florida, i used the app for giggles....Then I checked it with the compass inclinometer.....The inclinometer was more on target. Had I gone with the app, i would have been in a tree on the adjacent property.