Seems like a good point at first glance. A few observations though.
1. The Y axis needs to be wider because it supports the weight of the saddle and the table+vise+workpiece which can travel from one end to the other. In other words these can be large momenta twisting the saddle, so you need need wider base. The weight of the Z axis on the other hand is directed down so column torsion would only be created by cutting forces.
2. Commercial vertical milling centers do not seem to generally follow this rule. Some have almost the same spacing for Y and Z rails while others do not, like the one bellow:
Attachment 268858
I have a little wiggle space for the column, it is currently 10" wide at the top, I can make that 12" add add 2" between the rails. Another option is to make the head a little wider than the column so the rails can be moved further away from each other.