lm317 - How should I select the resistors for this constant-current application for a 317?

Over at OnSemi, they have a little doc about using a 317 voltage-reg as a current source: http://www.onsemi.com/pub_link/Collateral/AND8109-D.PDF

Their final example, providing constant current to an indeterminate number of strings of LED's is exactly what I'm looking for. I, somewhat, grasp how to choose the value of the "sense" resistor for each string, but they don't explain how to choose the resistor on the base of the transistor... and it's not completely clear to me what will happen if I choose a value which is too low, too high, and how to choose the optimum value.