When measuring for custom clothing, should you measure your body or clothes? Aaron Marino of alpha m. wants you to ALWAYS measure an existing garment.
Never measure your body. If you measure your body then proceed to the fit options, fabric is added to your body measurements which is not as exact as measuring an existing garment. If you don't have a dress shirt that is optimal, look where you would like it fit better. Reduce the amount you feel you need. When receiving it, you can take it to the tailor for tweeks. With jackets and suits, buy one at the store, measure it, and then return it.