An article today in the Huffington Post gives a better indication why Egypt claims the lion's share of of water coming down the Blue Nile. The simple reason is: it has no other sources of water. Ethiopia- which seeks to limit Egypt's share of Nile water- is the water capital of Africa. In fact its current policy of building big hydro dams to export power reflects this surfeit of water. Poor old Egypt only has old father Nile to rely on. The article goes on to show that how much water you get shouldn't be some childish 'equal shares for all' but in reality should reflect how many people absolutely need that water. If the people of Wales suddenly turned off the taps to Birmingham and demanded half of all the water they sent to that city we'd think it odd. Similarly its crazy for Ethiopia to claim more water than it needs- especially from a country that has no other source of the stuff.
http://www.huffingtonpost.com/2013/06/14/egypt-map-ethiopia-grand-renaissance-dam_n_3437528.html