It also depends on how literally you take the frase, " a player may always take a five foot adjustment...". Move, shoot, five foot step behind cover. Charge for 60', five foot step to the left, attack..., it could get ugly. Also, depending on what you are hiding behind, your cover is dependent on where the enemy is. A quick move around a pillar and the enemy has you in the open again, unless your taking a redied action to stay in cover, and then you won't be shooting anything.
I would say he would have to take a consistent amount of cover, or just deal with the fact that he could only ekk out full cover every other round. I'd say 3/4 for a bow, outside of an arrow slit, or 9/10 for a gun or cross bow, as the only things in the open are the weapon and your eyes. As far as melee goes, outside of a tower shield, both opponents are already considered to have cover equal to one another, no matter what it may be. You can try to stab someone thru a crack in a door, your just going to miss.