I used an old T shirt which fits me well and decided to copy it by laying it on a sheet of pattern paper placed over a blanket . I then used a needle to punch the outline of the pattern pieces onto the paper . Next I transferred all the measurements from the Tshirt to my pattern and then copied that pattern and added seam allowances etc. I made up a toile in a similar viscose rubbishy knit and the found that the shoulders were too wide , the sleeves were too wide and that the body was a little too wide . I also decided that I wanted to raise the back neck by about 7 cm as I find that having a high back neck is MUCH more flattering my body which has more than average sloping shoulders ..
I made these alterations and then transferred these changes to the paper pattern. It seems to me that I will have several different T shirt patterns made up to suit the different types of knits which I will want to use .Knits like wool jersey which are very stable may well need darts whereas the viscose knits don't .