Can't speak for the Norma, but based on the samples I have here in the office, the necks of a recent run of Lapua 260 brass is running right at .015". Given the normal .002" springback allowance, I'd suggest starting with a .292" bushing (.015 x 2 = .030 + .264 = .294 - .002 = .292), but I normally suggest also buying the bushings .001" above and below the basic figure. That is, the .291" and .293" bushings as well. Gives you a bit more latitude to accommodate dimensional variations in brass that you may encounter over time, and allows you to experiment with some slight differences in neck tension as well.
Besides, it's another gadget for the reloading bench, and we all know you can never have too many gadgets.