I played around with the constant current driver circuit I found on Pcbheaven, because it's build around a mosfet like I'm using
Boy, are you ever persistent in doing the wrong thing and completely ignoring the advice you get here.
I've calculated the RS-resistor by using the BC337's Vbe-value which is 1.2V following the datasheet.
You picked a number out of the datasheet without knowing what it meant.
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 :-)
Because you ignored my instructions in reply #17 and went and did something totally random.
The 1.2 Vbe (I am assuming your 2.1 above was a typo) is the Max on the datasheet, not typical. I actually suspect it is less than 1V.
Nothing to "suspect". I specified it in reply #17.
As far as I can see from the circuit in #33 to the circuit in #36 I'm using is the resistor at the control-line in #33. If I add such resistor (10k?) to my circuit, I should be good? :)
Don't "add" anything at random. Use the circuit we gave you.
[Yes, it makes no difference whether it is a transistor or a logic FET.}