R1 along with R2 sets the output voltage.......R1 is nominally 240 ohms....making R2 adjustable sets the output..........
The purpose of the 2R2 resistor is as follows :
Consider the output voltage as 13.8v, as the load current increases, the o/p volts drop, the reg makes up the difference in v out, the circuit then becomes stable. Now as the load current increases the voltage across the 2R2 resistor (R3 for arguments sake) rises, as V=I*R,
Now Vbe of Tr1 is ~ 0.6v, so if R3 is 2R2 and the load current rises such that it develops a voltage across it of 2R2*I volts, then as it approaches Vbe of Tr1, Tr1 will start to conduct, causing a rise in o/p volts, the reg cuts back its o/p and so the current falls, eventually the system restabilises.......
The value of R3 isnt critical it sets the point of operation of Tr1, the higher the value of R1 the more current is passed by the regulator until Tr1 turns......the ideal situation would be to allow a nominal current through the regulator to enable it to perform, and the remainder of the current supplied by Tr1........
Hope this explains it all 8)