I played around with the constant current driver circuit I found on Pcbheaven, because it's build around a mosfet like I'm using: Constant current driver
I built the following circuit:
I've calculated the RS-resistor by using the BC337's Vbe-value which is 1.2V following the datasheet. The rs-resistor is calculated by 2.1V/0.35A = 3.4ohm. (I used a 3.6ohm resistor because that's what I had on hand (2x1.8ohm))
The RG-resistor is just a 10k as it's stated at pcbheaven that it should be fine - should I also calculate this to be sure?
The circuit works great with PWM and all. But there's something I find a little strange... With the above circuit the led-pcb draws 160mA, I guess it should actually draw 350mA as that's what the RS-resistor is calculated for (to my knowledge). If I lower the value of the RS-resistor I can get it to draw more from the power-supply, but then the calculation for the RS-resistor isn't what I calculated it to be anymore.
If anyone knows why, I would like to know what can cause that my led-pcb isn't actually drawing 350mA when that's what I calculated the RS-resistor for 
Best regards
Joe