I don't recommend neck sizing for the M14 style actions. The usual rule of thumb is to use a full-length resizing die adjusted to set the case shoulder back a minimum of 0.002" (0.05 mm). This is mainly for reliable feeding from the magazine, but with the floating firing pin you also have the potential for slam firing if a case doesn't chamber easily enough, and there is some potential hazard associated with that as well as the possibility of doubling (a second shot following the first in fully automatic fashion). Especially when you are not using military specification primers.
Your groups sound quite reasonable. I have an M1A that will shoot 0.7 moa, but it is match accurized. For a gun that has not been worked on by a gunsmith, anything under 2 moa is a gift.
In changing powder charge by 0.5 grains, your steps are a little large and can possibly skip over a sweet spot load. I recommend 0.3 grains steps in the .308. You might like to take a look at this site
for a systematic means of finding best loads.
I found that for load testing with the M1A, the B-Square telescopic site adapter was useful. The gun shot best off the bench for me when I rested it on a bag just ahead of the magazine and not out forward on the stock. For non-bench, prone position with a military sling shot best. I found that with single-loaded rounds, having low bullet runout with respect to the case is helpful and can reduce groups by up to 1/2 moa with some bullets. For that reason I use a Redding Competition Seater Die when I load for mine.