Hi Jason, Ive been doing this work for so long that the placement comes automatically. Generally for the front and a 10cm x 10 cm design, 6 finger widths down from the collar and two across from the center, or find the point where the collar meets the shoulder and place one third of the design to the left of this point and just above the nipple.
On the back it depends on the design size. If it an A4 design in landscape then I find the point about 16cm down from the collar and place the design one third above this point and two thirds under. If its bigger then get someone to put a shirt on and you place the design on the back and measure how far down it comes from the collar.
Ok for M to XXL shirts , smaller and larger you will have to play around a little.
For exact measurements I use a ruler and a set square to put it squarely on the shirt.
The first shirt takes 2mins to set up and then the rest I use finger widths.
Earl