You should calculate the values for the resistor to suit your application. Before working out the values though, it will help if you can define the following:
ADC reference voltage and bit resolution: e.g. 5V, 10 bits
Output current resolution: e.g. 100 levels from 4mA to 20mA
PWM resolution and frequency: e.g. 8 bits + N bits dither @ 60kHz
Output current change rate: e.g. don't care
From these values you can work out the resistor values and how the current limit circuit will be configured.
Assuming that you want an analogue output current, and not a switching PWM, you should put some low-pass filtering on the PWM before driving the transistor (e.g. a capacitor on Q3 base-collector).