For Power Factor Correction, it is much more complex. The modulation scheme is intended to make the input current look like a sinewave. To do that, it has to look at the realtime input voltage waveform, the realtime measured current, as well as the output voltage. Then calculate what on/off ratio to drive the mosfets at for each pulse. Then measure and calculate it again for the next pulse. Again and again.
That's what the LT1249 and other active PFC control chips do. To do it manually, you'll need to emulate all of the internal functions of those control ICs.