I am studying phase angle control and I need to derive a simple equation that allows me to calculate the angle of a sine wave at a particular voltage, to trigger a triac.

For example: say i am given a RMS voltage of 240V and I wish to find the angle at which the instantaneous voltage is 65V.

I know at 0pi the voltage is zero and at pi is 240*(sqroot2) or 339V. But how do i find the values in between?

Many Thanks