Unless you are seeing pressure signs,or approaching max,I wouldn't necessarily take that loosening of group to be a "stop,back up"
I'd pour more powder in,so long as you are inside max,and not seeing trouble.
See what happens about 45 gr,or wherever your upper limit is.Note,Sierra shows a Max of 43.5 gr in the 5th ed.My 2011 Hogdon annual shows 46 gr compressed with the 168 gr MK.Brother is generally using Nos Custom Comps.
http://thefiringline.com/forums/atta...9&d=1316500028
That link is firing line,it will show you a 300 d target,three shots to check zero before antelope hunt.165 gr Ballistic Tips and someplace between 45 and 46 gr Varget in WW cases.Its not a fluke.
This is at 600,168 gr Nosler Custom Comp,Varget,2700 fps,20 in bbl
http://thefiringline.com/forums/atta...4&d=1307209204