Is is only true that "Average voltage= (RMS Volt/1.1107) , am I right ?" for a sine wave. When you have triac control of a load, the waveform is only a pure sine wave at a conduction angle of 180°. You'll notice that the red curve and the black curve coincide at 180°.
At any other conduction angle, the output waveform is most definitely NOT a sine wave. You understand that, right?
The actual average response of a non-RMS meter is multiplied by 1.1107 under the assumption that you are measuring a sine wave, and want the RMS value. If you're not measuring a sine wave (which is what is going on at any conduction angle other than 180°), then the multiplication by 1.1107 is not appropriate. That's why the black curve does not coincide with the red curve except at 180°.
One could use the formulas for RMS and average of the triac output waveform and calculate the ratio of RMS to average versus conduction angle (and also divide by 1.1107 to get a curve of the factor which relates the ACTUAL reading of a non-RMS meter to the true RMS voltage). Remember that the READING of a non-RMS meter is not the true average value of a waveform; it is 1.1107 times the average value, so that for a sine wave the actual reading of the non-RMS meter is the RMS value. This graph shows the result:
Notice that the ratio is ONLY equal to 1.000 for a conduction angle of 180°. For other conduction angles, it is different. In particular, at a conduction angle of 90° the calculated ratio of RMS to the ACTUAL reading of an average responding meter is 1.41421 (which I recognize is Sqrt(2)).