The usual place to measure from is the aperture - in this case the Waterhouse slot. I haven't found your lens in my references, but my first guess is that it's longer than 300mm - perhaps 360mm?

F-stops are rather simple to calculate, at least with longer focal lengths:

Note that f-stops are always given as a ratio - f/16 for example (or f:16, same thing with different notation). Thefis actually the focal length, so for f:16 the hole diameter should be 1/16th of the focal length! It's really a little bit more complicated since it should be the diameter of the entrance pupil to be absolutely correct, but in most cases the error with using it as hole diameter is minimal and would lead to less than half a stop overexposure.