For a high-gain (1.3) screen like my Stewart Firehawk, further back is better. The reason is as follows: A longer throw will minimize the angle of the light-cone, and this maximizes the reflected light back to the viewer positioned at zero degrees on-axis. In contrast, a shorter throw distance will maximize the light-cone angle and cause greater off-axis reflected light away from the on-axis viewer.
Secondly, a high-gain screen in particular (but any screen) will benefit from a ceiling-mounted projector positioned such that the viewer would "see it" in the center of the screen if the screen were considered to be a mirror. This way, the light from the ceiling-mounted projector reflects downwards, bouncing directly towards the viewer. In contrast, a low mount (as in a table-mount) will cause the light to bounce up towards the ceiling, away from the viewer.
I also have a Sharp projector. Ideally, the Sharp projectors should be mounted level in the horizontal plane, and at a height in which the lens is level with the top border of the screen. The vertical offset wheel should then be used to bring the picture down to fill the screen. It is not ideal to "point" the projector downward at an angle and then correct the distorted image using keystone. Try to avoid using keystone at all -- instead use proper initial positioning and the vertical offset wheel.