TRIAC Firing angle vs power delivered to load, how calculate?
See formula from Jony130 here: https://www.physicsforums.com/threads/how-to-calculate-rms-voltage-from-triac-phase-angle.572668/

On the other hand, if your load is not a lamp, but a slow resistive thing with huge current draw like heaters, you may better go for a standard zero crossing SSR. 

The solution is just like PWM (or phase control), except that it is slower. Assume you have a time period of 0.8 seconds (800ms), and you only turn on/off the load at zero crossing. Zero crossing triac triggering takes care of turning on, and the triac itself will turn off after a zero cross. Now, if you turn on the SSR for one half-cycle (8 ms) and keep it off for 99 following half cycles, you have an 1% power. If you keep it on for 22 half cycles, and off for 78, you have 22% power, and so on. 

This however assumes that the load reacts slowly, i.e. this trick will cause serious flickering if you're using a light bulb, or humming in case of some load. On the other hand this trick is quite friendly for slow loads, makes no transients and no unwanted harmoics on the power lines.