It seems we have several terms here -- what you call "dead space" is "powder space" -- that is, unused space in the case. In general, a full case tends to be a bit more accurate than one that has a lot of unused space. In some cases, the powder can take up so little space that the orientation of the cartridge before firing can markedly affect accuracy -- hold the weapon muzzle up, then bring it to firing position and you get one point of impact, hold it muzzle down and you get another.
"Freebore" is a gap between the bullet at rest and the lede (the origin of the rifling.) In some rifles, freebore is built-in (Weatherbys used to be famous for this.) In general, most rifles will shoot better if there is relatively little freebore -- that is, if the shoulder of the bullet (the fartherest forward full diameter portion of the shank) is quite close to the lede. Stony Point makes a comparator -- a measuring device that will let you accurately determine how close your load positions the bullet to the lede.
In most rifles, seating the bullets close to the lede is more conducive to accuracy than reducing excess powder space. If you think powder space is your problem, try shooting a dozen rounds over a chronograph -- one symptom of excess powder space is a wide variation in shot-to-shot velocity.
In your case, you have answered your own question, "Accuracy is increased if the reload is set to about .020-.040 from the lands." The gun is telling you what it likes.