Atheism as the null hypothesis.

[Jehanne]

Agnosticism is, of course, a statement about knowledge; atheism is a statement about belief, or, rather, the lack, positive or negative, of a particular belief.

Your statement above is quite correct.

A true agnostic would regard the existence of god, any god, to be a 50/50 proposition.

This is not correct.

There is nothing within any definition of agnosticism that requires any sort of probability calculation. Nor, is there really any way to calculate such a probability. 

To calculate probabilities, data points are required. I'd be very interested to know where your hypothetical agnostic acquired their data points. As in: how many universes did they study in order to determine how many of them have gods vs how many do not? 

The classic, formal definition of agnostic is, "a person that does not know, and/or believes it may be unknowable, if a god exists".

I see your point, and there are, of course, different flavors of agnosticism as there are different flavors of atheism.  I agree with Dawkins, however, (see The God Delusion, "Poverty of Agnosticism"), that a true agnostic would have no knowledge one way or the other, for or against, some type of theistic/deistic god.