Eric,
The instructions I was told was to use 375 degrees F, 45 seconds with extra heavy pressure. To get the extra heavy press, a swing away press was recommended. The yellowing might be scorching the shirt or the adhesive on the paper. Try droping the temperature down, increasing the time and load up on the pressure. The white spot in the middle might be a cold spot on the heating element of your press or the platten might not be perfectly flat. (These are just guesses since I have not seen your press).
Here is the instructions that I found on the ChromaBlast website -
http://www.chromablast.com/downloads...structions.pdf. According to this, your time of 40 seconds might be too long. Drop it down to 25 seconds and see if that works. Remember to do your wash tests. Best wishes.
Mark