The reason I brought up PFC is that; Most boost up switchers have a fast error amplifier. So fast that they will try to filter out 60hz noise. A PFC’s has a very slow error amplifier. It takes about 6 cycles of the 60hz for the amplifier to respond.
In your case you want to buck down 240 volts to lets say 120 volts (RMS). If you have a fast error amplifier, anything above 120 volts will be bucked down to 120. You want to run the FET at 50% duty cycle. The error amplifier will look at the RMS of the output voltage and adjust the duty cycle to maintain 120 volts AC.
If the input voltage moves the output voltage will move. In about .1 second the error amplifier will move the duty cycle around to cause the output voltage to remain constant.