I'm not sure there is a rule of thumb as to proper slope per foot of gutter. As long as the gutter slopes downhill, the water will find it's way. The steeper the gutter slopes, the more velocity the water will have, and the more likely it is to wash the debris in the gutter down to the downspout end. That can be good, it can be bad. if the downspout outlet is large enough to handle the debris (leaves, etc) and has no screen over it, most things will wash out. If the downspout outlet has a screen over it, the debris will all build up at one end of the gutter, and, of course, the water will end up just spilling out.
Depending on where the gutter is, a good thing to remember is how it will look once installed. Many times people will install gutters that slope more steeply than they need to, and the appearance of the gutter, as seen from the ground, as it slopes along the soffit fascia board can be less than desireable. It will really catch the eye.