One note is when you get into good flowing heads (like Indy's or Big Port Commando's) and big cams, it tends to get inaccurate. I replicated a 660 hp 426 LA that Horsepower TV built and it only made 590hp at 6000 and 580 lb/ft tq at 4500.
Thats was my point. It can show trends, but it wont give exacts. That, and the quality of machine work can make huge diferences in the final result. You can do every operation, and buy every exact part, and put it all together, and still not make what they say, or perhaps exceed what they say.
