How best to apply Blend If depends on the layer it is applied to, and there can be lots of variations. For example, if Blend If is applied to an adjustment layer with a layer mask (i.e. selection) already, how you adjust the sliders can be dramatically different. Also, in addition to Gray, Blend If can be applied to a single rgb channel. The possibility is endless, and there is no single "best" way.
Unfortunately, this great feature is seldom addressed well in most of the PS books. Hint for those looking for a good PS book: flip through the chapters and index and see how many pages are dedicated to Blend If.