Militarily speaking, you want a straight line formed by the shirt front, your belt buckle and your zipper. What I mean by this are the parts of the shirt and zipper than are the “flaps.” (This is called a gig line.)
Also, in addition to tucking your shirt in tightly (within reason), you can get a more uniform appearance if you grab evenly whatever is billowing at the sides of your waist/hips. and fold that extra material (usually and inch or so) toward your butt. The spot where you make those folds should be at belt level or just below, so that they stay put.