A common worry is how one should adjust for shooting at "extreme" uphill/downhill angles. Chairgun was used to create a table of hold unders needed for various angles and distances. Looking at this table will explain the development of the Airgunner's Rule (apologies to the Rifleman's Rule): If the angle is less than 20 degrees or the distance less than 20 yards, shoot for the center. Otherwise, aim for the bottom of the kill zone.
Note that one shoots lower for angled shots whether the angle is uphill or downhill. Also, unless the angle is pretty extreme (and 20 degrees will feel like shooting straight up or down), one does nothing.
Introduction to Field Target by Joe McDaniel. This was created as course material for new shooters interested in field target at DIFTA.
Ways of the Rifle by MEC. Excellent book aimed at 3-Position shooters (only standing and kneeling match FT positions), but the approach to building a position is excellent reading for very serious FT shooters. Copies available from Champion Shoters, Creedmore, and others.