As you form it up (larger in diameter), the brass gets thinner (stretch a wide rubber band and watch it get narrower). If you form the old shoulder out very far, it may become thinner than the original neck.
To calculate the change, first subtract the area of the inner circle (inside diameter) from the area of the outer circle (outside diameter) of the original shoulder at the point where the new neck will begin. The difference will be the cross-sectional area of the brass at that point. Then calculate the area of the larger inner circle (after necking-out) and add the area of the old cross-section to see how large the new outside diameter will be. There you'll have, at least theoretically (in the difference between the two new diameters), the wall thickness to expect after you've formed the larger neck.
Let
A = area,
D = diameter, and
R = radius
in each calculation of the area of a circle.
In each such calculation, A = 3.141592654 � R � R
and of course R = � � D
