A method that generalizes the notion of frontier orbital (FO) theory is introduced. The method is based on the projected reactive orbitals (PROs). Although PROs have been shown to describe local reactivity better than FOs in high-symmetry systems, the PRO method needs an arbitrary choice of a reference atomic orbital (AO), causing ambiguity of the method and poor applicability to low-symmetry molecules. To overcome these difficulties, we examined three different kinds of methods for uniquely determining the reference AO, one of which (Method 1) was reported by other authors (Kurita, Y.; Takayama, C. J. Phys. Chem. A 1997, 101, 5593-5595). We specifically applied the methods to the prediction of basicities of heteroaromatic amines. The study showed that the newly developed reactivity-index maximization method (Method 3) yields the most reasonable PRO.