I don't get the point. Say an average .40 magazine holds 12 rounds. How much weight are you really reducing by only putting 6 rounds in it? How much do 6 .40 caliber rounds actually weigh? A .40 round weighs between 6 (105gr) and 13 (200gr) grams depending on the grain you choose. So lets say the average weight is ~10 grams.
~60 grams, that is all you are reducing. I say fill the mag and forget it.
Brain (Use this one the most)