"My ultimate rule is 'know thy tailor.' I'll tailor anything, I'll put darts in polo shirts. I have no problem doing it; alterations are key. A lot of times gauging what fits you properly depends on how you feel in it. If it feels tight, if you feel restricted, or if you feel sloppy and dumpy, then it's not right, it needs to be fixed.
I always tell guys they should know their barber, practically have him on speed dial. You should know your tailor like that as well. You should have one person that you go to who understands your body and understands how things fit on you. That'll create a consistent silhouette for your looks."