RE: MT Flo. Orange Requirements
A vest should be far more than 400. width across shoulders is at least 15 inches. Length from shoulders to waist is probably at least 20. That's 600 square inches for front and back. Doesn't include either side. You should be good to go.
The 400 inches is probably specifically made to allow for simple front/back 3/4 length pullover type safety vests.