I read this whole thread, i probably missed something, but im still unclear as to what would leave the smoothest texture between 3/8 and 1/2 or 3/4 for acrylic? And doesn't going over paint with a dried roller help create orange peel? I read once (prob on this forum) to go over your paint as little as possible after its on the wall evenly.
The general rule is the smaller the cover size (nap) the smoother the wall finish, partly because of the covers paint load capacity... but everything is relative, like the sheen of the paint, the consistency of the paint, the amount of paint applied to the wall, the type of cover, the smoothness of the wall and your ability to lay it off evenly.
Once paint is applied it cannot be taken off it has to be rolled out, and It is possible to apply too much paint to a section of wall, especially if you are applying a sheened paint to a sealed wall, IMO in this case a 1/4" - 3/8" cover (dependent on the level of sheen and smoothness of the wall ) will give you the most control while laying the finish off and will give the smoothest finish.
Flat paints usually need more volume applied to the wall, IMO a 1/2" - 3/4" cover works well and limits "dry rolling" which can be a real problem when repainting an existing flat finish.
As Brush sez.. it really does come down to a personal technique, which comes with experience. Experiment with different covers... and have no fear, you'll find a few that fit your style...
rmichael