Common Collector (as drawn):
Assuming fan is a constant resistance, 12v/0.21A = 57R
If the transistor is turned fully on then you have a 68R resistor in series with the fan.
57R/(57R+68R) = 45.6% of 12v = 5.5V available to the motor.
The series resistor is not required anyway, the motor will be the load for the transistor, it only takes 0.21A with 12V across it so there is no way you will see 0.5A through the transistor!
A silicon bipolar transistor will drop 0.7V between base and emitter when it is biassed on so to get the maximum power to the motor you need the base voltage to be as high as possible and the emitter will try to follow (with the 0.7V drop). If you connect the base directly to the 12V supply then the transistor's emitter will rise to 11.3V (this configuration is also called an emitter follower as the emitter simply follows the base voltage!).
If we stick with the assumption that the motor has a constant resistance of 57R and you say 0.048A is the minimum current you require then this equates to 57R*0.048A = 2.736V
To make the emitter control at this voltage we set the base at
2.736V+0.7V = 3.436V
So the base resistor needs to drop 12v-3.436V = 8.564V
With a current of 0.048A/400(hfe) = 120uA
So R3 (the base resistor) is 8.564v/120uA = 71367R or 71k4
This means R3 needs to vary from a dead-short to 71.4K