With 4064, in particular, it often is the case that's is not too sensitive to small load variations. The reason for 0.3 grains is partly that it's the smallest measure you can make on a scale with ±0.1 grain accuracy that you can be sure is actually different from the previous load. The other is that it comes from Dan Newberry's OCW approach of using about 0.7% to fire his round robins where he identifies a sweet spot by finding a load range where the point of impact doesn't change for three loads (a 0.6 grain span). That makes the midpoint easier to identify. But there's nothing to say you couldn't have a wider one. At the extremes, I've seen an accuracy load that was completely opened up when I went .5 grains either side of it (this was in .308), and at the other, a different load that stayed right in there for a span of 2.5 grains. (That was using Brigadier 3032, so naturally the plant burnt down and you can't get it anymore.)
Gotcha on the load workup. Since both were BK's you weren't likely to run into construction differences that raised pressure as can happen when switching the bullet type or make. Still, you might have a low load somewhere down there that shoots well for you.
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