Those formula's are the ones that most of the online calculators are based on.
They were originally derived based on hard science, and then tweaked until they produced results consistant with the real world.
(edit)They appear to have been derived in the mid 1980's
They seem to be pretty accurate as long as your dealing with typical cars. They probably lose some accuracy in extreme cases.
For example they totally ignore aerodynamic hp, at least they use an assumed value for it.
They would give the same result if the vehicle is a minivan or a motorcycle, even though there is a huge difference in frontal area and CD.
A full size Cadillac of the early 1970's had about 115 lbs air drag at 70 mph, while the Ferraries had an air drag of 57 lbs at the same speed.
The results are probably most accurate for cars like the Mustang, Camaro, and full size older cars. It should over state the HP for smaller modern cars that have much better aerodynamics. The .32 CD on the WRX is actually quite good when compared to the typical .45 of the late 60's early 70's before fuel economy became important.