Scattering resonance is a fascinating phenomenon which manifests as a peak or a dip in an observable as a function of collisional energy (Ec). Its occurrence requires a potential well to support the resonance states. In this regard, reactive resonance is unusual in that it can exist in a reaction with unbound Born-Oppenheimer potential energy surface, on which the quasi-bound states are dynamically trapped-meaning that some energy is temporarily tied to the other degrees of freedom than the reaction coordinate. The concept of vibrational adiabaticity has been the cornerstone in understanding this phenomenon, for which the vibrationally adiabatic well depth is of primary concern. Recent studies on the F + CH3D reaction have accumulated compelling evidence for a dominant resonance-mediated pathway at low Ec as well as for a rainbow feature in pair-correlated angular distribution at higher Ec. Here, we report an in-depth study to not only substantiate both claims but also, more importantly, make the first attempt to quantify the vibrationally adiabatic well depth directly from the observed rainbow structure and then compare with the theoretical prediction.