I'm not familiar with the Optoma, but keystone is normally needed if the pj is placed outside the range of the lens shift, maximum ranges. If your pj is placed within those ranges, in relation to where the screen is, you shouldn't need to use the keystone feature.
It looks like the 14'9 is referring to the distance from the screen, and as you change the gain, it will change the optimal distance from the screen, due to brightness either lowering, or increasing.
This isn't necessarily related to the use of keystone, unless your placing the projector at say, 15' away from the screen, but also 10' ABOVE it, which would be way outside of the capabilities/range of the lens shift feature. That is an extreme example, but might get the point across I think?