4-20ma output from Arduino

I'm controlling heater. I understand, that I can simply take 60sec and turn it on 3-10-50sec, but that is not so good move, because when heater is fully on, it give too much heat, so the product burns.

I'm not good electronic, and don't understand fully "phase angle control". I just need to make heater work not on full power (some time 10%, 30%,..100%)