Hey Doc, look, I'll level with you. The sales of your latest book? They're not great.
Getting the perfect fit is easy. Just grab your favourite tee shirt, lay it flat on a table, take a few measurements and pick the closest matching shirt. If you are in between 2 sizes always go on the larger, chances are your favourite tee has shrunk a little since you got it. Shirt sizes can vary by +/- 3cm, and will shrink by 1-3 cm over time.