This site should help you.

Here is a link that might be useful: Embroidery Placements

This guide is good but with 1 1/2 to 2" spans, you still have to guess some. I have the Embroidery Buddy placement thing. It is in a backward L shape from center front of shirt to placement & from shoulder to placement. You could take something like poster board & make one. It is sized for women & mens sizes. Large woman & med man about the same.

Make it about 7" wide & 10" high... in a backward L. Use the 7 1/2 down for the Smaller & 9 for XL & mark a M & L inbetween.

Do the same for width with 4 for S & 6 for XL, Mark M & L inbetween. go lower for XS & higher for XXL & XXXL.

The outside corner of your L is where the center of your design should be.

I, too, have an Embroiderer's Buddy which really helps in placement. However, I also have a "Monogram Placement Guidelines" sheet from Embroidery Online that says for golf shirts, place the monogram 7 1/2" to 9" down from the left shoulder seam and 4" to 6" over from the center front of the shirt. The accompanying diagram indicates that by "left shoulder seam", they mean right where the collar is sewn on at the shoulder seam. I would use the smaller placement measurements for the smaller shirts, graduating up to the larger placement measurements for the larger shirts. Scale it down for children.

The guideline sheet also gives an alternate way of determining the correct placement of a left chest design on any size shirt. You would draw an imaginary line vertically from the top of the shoulder down and then draw another "line" horizontally through the middle of the sleeve (this would be for a short-sleeved shirt). Where these two lines intersect is your correct placement.

Good luck and post a pic when finished.