Precise Current Limiting for Variable 0–300VDC Supply
Q2 limits current by pulling the Gate voltage down when Vbe gets to ~0.6V. This corresponds to 50mA when R2 = 12&ohm;. However due to the logarithmic response of the Base-Emitter junction the cutoff is quite 'soft', starting to limit at about 40mA. Another potential problem that the junction voltage has a temperature coefficient of about -2mV/ºC, so the current limit will reduce as the transistor's temperature rises.

You could replace Q2 with a TL431. This device has a precision 2.5V reference and comparator connected to a bipolar transistor. It should precisely and sharply limit current to 49mA with With R2 = 51&ohm; (51&ohm;*49mA = 2.5V). 

Here's the circuit:-







simulate this circuit &ndash; Schematic created using CircuitLab

The TL431 is very cheap and readily available. If you have old switch-mode mains power supply that you can rob for parts then you might even get it for free!