WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
I would take your current "sense" resistor value, multiply it by 2.25. Using that calculated number for resistor value, I would then parallel a fixed resistance and trim pot of same value to allow you to adjust it precisely to the 2,00 mA you desire. I used 2.25 to give some leeway +/- on the adjustment. You can then measure the combined resistor value and replace with a fixed or leave it as it is to allow you to adjust as required.
It doesn't matter if it's not constant as long as it's large- see DF96's analysis. Unfortunately, the DN2540's impedance is pretty mediocre at low currents. For the 2mA you're seeking, a bipolar cascode will perform much better.