Welcome to the forum Rusty - I doubt you're really "Old" ! I'm sure that one of the Sikorsky guys can help you with the rated thrust of the X2's prop. I used to know one of the program pilots Kevin Bredenbeck so maybe you can try his name in the standard Sikorsky email format;
[email protected]. Most of those guys are quite bright and also like to share their love of rotorcraft knowledge with others so he will probably give you more than you want or even need to know !
As for the formula; I used to have it in my old college aero textbook but its long gone. In simple terms, its just geometry and fluid mechanics with a few other minutae. Google is your friend - don't need to pull out the old textbooks anymore :-)