The 75% ray is the angle from the onaxis ray that gives a 25% obstruction of the whole beam as defined by the diameter of the primary mirror. There is a circle of full illumination (100% of the light coming into the scope and hitting the mirror) and then NEWT also defines a circle of 75% illumination which is pretty much what the designer of the software desired as the limit of useability of the scope. Obstruction larger than this value tends to be rapidly increasing as you're starting down the steeper part of the slope at this point. In other words, you will never get the 75% and the 100% together until you make the secondary so small that they are both onaxis and that's a very small secondary!