An expert in the field helped with the issue. The unit of angular dispersion for a reflection grating is nm-1 when the spacing of the rulings is nm. That makes perfect sense given the units used in the equation. To convert to radians/nm, you need to multiply the result by pi radians (half a circle for a reflection grating). This detail was left out of my textbooks and no examples provided. I want to express my sincere thanks to the person who helped to resolve the confusion. Jay.