IIP2 Second Order Intercept Rule of Thumb
If you know the other variables, you might be able to solve for IIP2.

Second order intercept point in cascading stages.   

1/sqrt(OIP) = (1/sqrt(G2 x G3 x OIP1)) + (1/sqrt(G3 x OIP2)) + (1/sqrt(OIP3)) 

IIP = (OIP/(G1 x G2 x G3))