A basic rule of thumb I use is
Incline: Add 1 foot of power for each 1 inch of incline. If more then five inches add a minimum of 6 feet. If more then 10 inches add at least 12 or 13 feet. (this helps account for drag)
Downhill: Take away 1 foot of power for each inch down, but like above, you need to make sure you dont under hit. So a 5 inch downhill at 15 feet, I would hit at 10 foot of power. A 1 foot drop at 30 feet, I would actually hit with nearly 20 feet of power.