Just checked it and it works too. Incase you're interested, I'm using a PIC micro to detect the zero crossing point of the domestic mains. I'm using that equation to calculate how long it takes for the Pic Micro to detect the zero crossing point on the falling and rising edge, given that a schmitt trigger is used as the input buffer (4V and above is logic 1, 1V and below logic 0).
With that in mind, I've worked out that the pic micro will generate an interrupt 37uS after the zero crossing point has occured on the rising edge and 9.4uS after the zero crossing point has occured on the falling edge.
Thanks again both
Brian