How it should fit
The preferred fit of the midsection is a matter of personal preference. If you prefer a more relaxed fit, you can make the midsection as loose as you like (although you won’t be able to go much larger than the chest width of the shirt). If you desire a tailored, slim fit look, you’ll want the midsection of the shirt to sit fairly close to your body, but not be so tight that it causes pulling at your stomach.
How the measurement works
The midsection of the shirt is measured across the center of the middle of the shirt body. This is at the point directly between the bottom of the armpits and the bottom of the shirt side seams. Modifying the midsection width by 1” adjusts the shirt circumference around your body by 2”.
We try to account for some shrinkage with how we cut the shirts at the midsection. Right out of the box, the midsection width should (very strictly) measure .25” to .5” wider than it is specified. We do this consistently on all shirts (except for high shrinkage fabrics where we add more), so generally speaking it is better to adjust relative to the first shirt you have rather than re-measure and specify a totally fresh number.