Is it Rhombi by Terri Kruse? That's a top down raglan and quite easily adaptable. Choose one of the sizes based on the neck measurement, and just work the increases until the back stitches measure half the 33½" measurement. Or round it up to 17" to give you a measurement of 34" around. Translate the sts that you cast on for the size you'd make into a neck measurement then see which of those sizes is closer.
sue- knitting heretic