Meat selection is a good place to start. You want to be as close to the same size/weight/thickness as possible after trimming. Judging the amount of meat/fat/bone lost to trimming looking at the package helps. The same goes for judging marbling. The more consistent the product is before it goes on the cooker the easier it will be to manage timing.
They will still cook differently, but you shouldn't have as much variation. When you start wrapping within minutes of your target, or whatever step you choose, you are establishing a routine. Then it's much easier to adjust timing during the cook.
Give it more than a cook or two. If you change something major every cook or two, it's going to be hard for you to get the consistency you are looking for.