Measuring Tips to Assure The Best Fit
Measure the circumference under your armpits, around the fullest part of your chest. The tape measure should be snug. Not so tight that it constricts breathing, but not so loose that the tape measure slides down.
From the top of your shoulder, follow your body down to the point where you want your shirt to end. Think of a line going from your armpit straight upwards to your shoulder. Measure between those two points and hold the tape measure straight.