We present a new procedure to calculate rate constants from reduced dimensionality reaction probabilities. The method combines an energy-shifting correction, as used in reduced dimensionality calculations of three and four-atom reactions, with a generalized transition state. The procedure, in combination with a recently developed reduced dimensionality model, is used to calculate rate constants for CH4+O(P-3)--> CH3+OH and its fully deuterated counterpart. These rate constants are compared with the ones obtained using the standard technique and with experimental values. Also, we study the effect on reactivity of exciting selected modes of methane. Similarities and differences between the deuterated and undeuterated reactions are discussed.