FSCL = Fclock/(16+2*TWBR*Prescaler)
However PFC8574 is sold as a 100 kHz IIC device .................In the mean time let me know, what do you think about this whole "fight for I2C overclocking"? Are there any possible sideeffects?
I have also created a dump of the signals, and compared the 100kHz, most reliable signal, to the 727kHz, least reliable acknowledged one. They show 100% match.