I would start by checking IF the shirt is straight.
What shirts and fabric type are you using?
T-Shirts for the most part are made of KNIT fabrics. Knit fabrics have stretch, some more than others depending on the knit...like rib knit will stretch more than jersey knit.
What I used to do before printing a shirt, I would lay the shirt down on the press, look at it and see if it looked straight, if it did not I would gently stretch the shirt into place, arm sleeves included.
Mind you that with poor quality shirts, nothing will help them being straight.
Give it a try, once the shirt is straight on the press, then use the tool to line up your graphic.
...Im sure someone with experience with this tool will come along and try to help your urgent need....soon.
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