Originally Posted by Despos
If you have a 40" chest and add 3" to the circumference of the shirt chest you will get 21.5" when you measure the shirt the way described. Total measure is 43", this is a very "snug" fit. This may be way too tight for some or just snug to others. Depends on your tolerance for a close fit. 40" chest + 2" would be a very tight fit. Not recommended. A deep breadth would pop a button.
Thanks for this Chris; really helpful.
If I may, a followup: my understanding is that the answer to this general question (chest clearance) is a bit larger for a shirt than for a jacket (which should generally be 2-4" for a good fit?). If I understand this right, is this simply because the shirt needs to accommodate a wider range of movement (sitting, standing, etc.) than a jacket (which can be unbuttoned)? If I have it wrong, please correct.