Hi Kerim,
Yes, I thinks this is a disadvantage of MOC.
As your formula, when I use 22Ohm resistor, and I want to get a current 50mA across optocouple, the voltage trigger must be bigger than 22*50 = 1100 mV, with a 220VAC-50Hz source, we can calculate minimum value of x :
The voltage at t(s) will be :
220*sqrt(2)*sin(314*t)
To trigger triac, voltage must be bigger than 1,1V, so we have this equation :
220*sqrt(2)*sin(314*t) = 1,1
=> t = 6,45*10^-4 (s) = 0.645ms.
So minimum value of x will be :
x = 180/10*0.645 = 11.61 degree.
So the open angle range will be x to (180-x) ~ 11.61 to 168.39
But note that when you use another way to trigger triac, you'll never get full load (0-180 degree), because triac (thyristor also) have a parameter named Ihold, current pass triac must be bigger than this value.
By the way, your formula : Rs = I_max / V_peak, it's not correct