Another "Hard and fast" rule to measure... The way that "After Hours" (a tux rental place) sizes you is like this:

take the chest measurement at the widest part of the chest and add 2". Write that # down.

Take the measurement around the chest AND ARMS at the same spot (the widest point) and subtract 6". Write that # down.

Those numbers should be BASICALLY the same thing. If they're not, take the bigger of the 2 numbers and that's your jacket size.

Is it custom or tailored? No. Is it going to be pretty much accurate (for your AMERICAN Tux size) Yes. For a Kilt Jacket, I'd sugest the +4 / -4 method as opposed to the +2 / -6 method.