In itself, the center of a shirt can be determined quickly. You fold it in half along the length, checking that the side and shoulder seams are level and throw it under the press for a few seconds. You then have a nice line across the middle (and your shirt is also pre-pressed in 1 movement). With Berlin Textildruck this is the best deal.
In combination with a transfer on which you have indicated the center with a crease above and below, you can perfectly center your transfers on the conveyor belt on clothing. Take extra care that you put both side seams and shoulder seems perfectly together and fold the shirt nicely flat, otherwise your folding line will not run perpendicular to the shirt.
The Alternative Method
An alternative method is to lay the shirt open and properly align the shoulder seams on 1 straight line. Parallel above that …