Mike,
I would guess the formula at zero forward speed would be:
Pi x D x rpm. If you assume a prop diameter of 2m and an rpm of 2000 this works out to around 720kph.
If you are doing a forward speed of 300kph, then by the velocity triangle (Pythagorus) the tip speed is around 780kph.
But I left school 30 years ago, so waiting to be corected on this one!