Steve,
Good call. I just assumed he had the number from Nosler, but my Nosler book just lists 2.260" for all their bullets. The
Lengths file at JBM says the Speer 50 grain Soft Point is only 0.620" long, while the Nosler BT is 0.800" long. Also, the Hodgdon data uses a Winchester SR primer. Others can be warmer or milder.
It looks to me, from QuickLOAD, like you not only want to seat the Nosler all the way out at the maximum 2.260", but then will need to reduce the Hodgdon loads about 2.5%, assuming a WSR primer and Winchester case. The longer bullet eats up some powder space, which accounts for this result. So this becomes 26.3 grains as a starting load to 27.8 grains max.
If you were using, say, a Remington 7½ primer, you might have to knock another 4% off those numbers, coming to 25.2 grains to 26.7 grains maximum. Same with a CCI 450 and #41 magnum primers (formulated specifically for St, Marks spherical powders).
I believe these loads are all well above 70% case fill, so no underloading pressure issues should be involved. My advice, if you don't know how your primer compares to the WSR and are not using a Winchester case (though case differences in .223 are small), is to start at 25.2 grains and work up toward 27.8 grains while watching for
pressure signs.