Here's a cool tool (
COOL TOOL) to establish overall straight line angles if you're averse to the whole Trig thing. As I deduce from the Cal State paper by Feysal Adem, anything between 5° and 12° will yield a great Cd reduction, anything closer to 5° will yield a significant reduction in lift which will help make for a more drivable truck. So using the tool, it looks like 8° will be 8" off the tailgate.
You do plan on curving the top I trust. Just going with a straight flat top you may want to go with 10-12" off the gate.