If you are decorating across a broad size scale ( Youth Small to Adult 2X-6X) you may need to create two sets of screens at differs sizes. For example, a 9.5" version and a 12" version.
QUICK REFERENCE GUIDE
Most fronts should be placed 2" -3.5" ( 2-3 finger width) below the front collar.
Horizontal Reference Line- The chest line is an imaginary line that runs across the center the chest connecting the bottom of each armhole ( where it intersects with the side seam ).
Vertical Reference Line- Center Line. Most t-shirts have a fold crease down the center of the shirt.
The focal point of the design should be centered on the intersection of the center line and the chest line.
by Proportional Height
Vertical Reference Line- Imagine a vertical line starting where the neck opening meets the shoulder seem
Horizontal Reference Line- Imagine a horizontal line running across the chest from the bottom of the arm hole ( where it meets the side seam to the other arm hole.
Center the left chest design where these two lines intersect.
Size landscape designs ( wider than tall) closer to 4" wide so the design doesn't appear too short.
Size portraits designs ( taller than wide) closer to 3" wide to avoid appearing too tall on the shirt.