How to calculate current sink
The 2N3904 base is at 0 volts and the emitter resistor connects to -5 volts. Given that the base emitter junction can be approximated to a 0.7 volt drop, the voltage across the emitter resistor is about 4.3 volts. This means that a 470 ohm resistor will have about 9.15 mA flowing through it.