Power on a slope

Im trying to figure out how much power is needed to maintain a vehicles speed on a slope

I found out that my car weighing in at about 2000 lbs needs 15hp to maintain 75 mph on a flat road with no wind. now I'm trying to figure out how to take that and calculate the power needed on a 10% grade and a 20% grade. (more looking for the formula than the answer)

I know that a 10% grade = 5.7° and 20% = 11.3°
tan^-1(grade)=degrees

The formula I used to find power needed to maintain a given speed works by taking the time it takes to slow a car from one speed to another in neutral (80mph to 70mph) gives me the hp needed to maintain 75 mph.

Heres the formula I developed a few years back and has been surpassingly accurate as it account for most all the losses in the tires and drive line rear of the transmission.

( [-6*10^-5]*m*[V_i+V_f]*[V_f-V_i) )/t = hp

m = mass in lbs
V_i = Initial Velocity (mph)
V_f = Final Velocity (mph)
t = time (sec)

Now, how do I apply a slope to this?