I don't understand why you are feeding Lb=0 into F2-2 when (a) says that LTB doesn't apply when Lb<Lp. F2-2 only applies when Lp<Lb<Lr
Regardless, the end of F2-2 says <=Mp, so if the equation does equal more than Mp, then you still use Mp. This is meant for the case of Cb > 1...