Dear @Kashyap.Subham,
In FAST v7, points R, E, G, and T are all aligned with the pitch axis for the undeflected blade. And theta_E and theta_I are aligned (referred to as StrctTwst).
Regarding AeroCent used in FAST v7, here is my description (from: NREL 5MW Rotor Geometry - Wind & Water / Rotor Aerodynamics - NREL Forum):
AeroCent in FAST v7 is used to locate the reference point relative to the pitch axis for which aerodynamic (lift, drag, pitching) coefficients are defined. AeroCent in FAST v7 is defined as follows:
AeroCent = 0.25 - [ (fraction of chord from leading edge to actual pitch axis) - (fraction of chord from leading edge to aerodynamic coefficient reference point) ]
(The 0.25 in the equation above comes from FAST v7’s assumption that the pitch axis passes through 25% chord.)
Best regards,