The thing with NEC's is, in order to NOT stress the convergence and focus circuits, you pretty much have to live with not using as much of the tube as most manufacturers. Maximize your size with the largest source you have, then do the raster centering. Make sure you center your test pattern inside the raster too. One can do things to use more of the tubeface on an NEC but if one does this, they run the risk of physically damaging circuits in the projector.
There seem to be two distinct schools of thought about which approach is best with NEC's represented on the forum. I am in the school which says DO NOT frig with the factory spec's, and accept a smaller raster when compared to another manufacturer. As such, yes, the half inch or so all around is what you end up with. You can be sure of a good setup position by maximizing phosphor use FIRST, then finding the physical position where this size fills your screen. A little more tedious, but perhaps it puts your projector in a slightly better position to use more of its potential.