Common sense folks...or maybe it's just me used to being precise because of quilting (humor here

and those that quilt will understand)
Okay...do not TRUST the center crease. I would say 98% of the time it is correct.
What I do is I press my shirt top and bottom because I do not like wrinkly shirt.
For center transfer, do your pre-pressing.
Next, pickup shirt and sorta fold (you're really just picking up) in half matching top and bottom shoulder seams. If image is full front then match bottom sides.
Then place shirt back down just FINGER PRESS a small area for your center crease. If you can't take the heat...then use a Speedball Brayer or a clean old t-shirt. That's your center mark. Because you have lifted the garment up and have let it hang (so to speak) and matched seams...it will be close to perfectly centered.
CENTERING TRANSFER...
Easy, bend in half and pinch top and bottom to mark center. You are centering the image...not the transfer paper.
If it's text...you can memorize the letter that is the center.
MEASURING...
I measure from the bottom collar seam with a Westcott drafting ruler that has a cork bottom. No sliding. Also handy if you need to "lift" a corner of
a transfer. Make sure you place your ruler always on same side (I place right of the center line). Now your ruler acts as a "t-square" if you use the lines on the ruler.
TOOL SUGGESTED
Embroidery Buddy ruler. I plan on getting one because I'm getting into pocket position designs.