HOWEVER, what I didn't realise is this applies to Full Format (FX) cameras and lenses. My equipment D90 and D7100 is the smaller DX format (APS-C sensor) which covers a smaller percentage of a full frame image, effectively giving a 1.5 multiplication in focal length. Put simply a 100mm lens in DX format is equivalent to a 150mm lens in FX format.
SO that means when shooting handheld with a 100mm lens in DX format you also need to use the 1.5 multiplier factor. So minimum shutter speed should be 1/lens focal length X 1.5. E.g for 100mm lens 1/150 second, for 50mm lens 1/75 second, for 200mm lens 1/300 second, etc.
NOTE that this is a guide for minimum shutter speed, handheld. A slightly faster shutter speed should give better results. Using a VR (image stabilization) lens should improve results.