OK.. I just played with some numbers and I'm fairly sure it is..

fps = TicksPerSecond / ( CurrentTime - LastTime )

Becuase if TicksPerSecond = 1000, LastTime = 12000 and CurrentTime = 13000

1000 / ( 13000 - 12000 = 1000 ) = 1

If the frame takes 1000 Ticks, then we are running at 1 frame per second.