The only test is experience. That stuff about obscuring the front hub may or may not work for you, but dont use it as a guide.
The "feel" you should be aiming for is a sense of relaxation in your shoulders, a bit of flex in your elbows, and a straight back at a comfortable angle. The exact angle is a personal choice but should vary between horizontal for all-out time trialling, to 45 degrees for relaxed touring.
Pick a good shop who will let you swap out stems. Use front-loading stems to make the process easier.