Mike, first you have to realize the max pressure for a 12 ga. 2-¾ or 3" shells is 11,000 LUP. It's important to follow load data from manuals to the letter, as already mentioned.
Powder bushings most always throw light, the same as bushings in handgun/rifle do. This is a built in safety factor, those loads with as much as 1.0 grains light still work well. The shot simply doesn't have quite the listed velocity that the manual says it has. Use of a scale tells you to use one number higher powder bushing to get to the listed powder charge.
Hornady shotshell loaders have shot bushings that are marked for specific shot sizes, they will throw exact weights of say #8 shot, 1-1/8 ounce of chilled lead shot. However, if you use magnum shot, which is harder, thus lighter, it will throw a few grains lighter shot charge.
Another factor is the type of loader used. Progressive loaders throw lighter powder charges from the same bushing, than the single stage loaders do. The reason is the progressive throws a powder charge for each handle stroke. The single stage, like a MEC 600 JR., throws a charge after the handle has made 4 complete strokes. That shakes and compacts the powder in the hopper/bottle to pack the powder into the bushing.
Lyman's bible/shotshell handbook will have lighter loads in it for all gauges. Load those specific loads to do what you want, but simply reducing the powder a little might result in a punky, funny sounding report, and extremely slow velocities.