The perfect fit of a dress shirt at the chest is a matter of personal preference. If you prefer a more relaxed fit, you can make the chest as large as you like. If you desire a tailored, slim fit look, you’ll want to reduce the chest width as much as possible without going so tight that it impacts comfort or produces unattractive pull lines across the chest.
How the measurement works
The chest width of the shirt is measured from the bottom of one armpit to another (roughly nipple height). Modifying the chest 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 chest width. Right out of the box, the chest width should (very strictly) measure ¼” to ½” 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 your first Proper Cloth shirt (after giving it 1-2 washings) rather than trying to re-measure and specify a totally fresh number.