This measurement is not a circumference measurement. Rather it is a width measurement, taken across your chest (front and back) at the point where the armhole would nip in furthest. On the front, this is approximately 2/3 the depth of the armhole, and on the back it is approximately at the half-way point of the armhole.
Place the measuring tape on your front above your full bust, at the depth where your armhole nips in to its greatest amount.
Measure the distance between armhole to armhole, at the point where the sleeve would nip in the most.
Repeat for the back.
Record the measurement.