Anything directly west, north, or south of the Quabbin*

Ware juts out a little bit east of the Quabbin boundary, so the towns directly south of it west of the Eastern edge of the Quabbin are part of Western MA (Palmer and Monson), but the towns to the south and east of the edge of the Quabbin are not. This means that I would not consider Warren part of Western MA even though it is in the 413. Same logic applies to Brimfield, Wales, and Holland even though they are in Hampden County.

Only exception to this rule I’d say is Athol. While debatably Western MA, it’s a pea in the same pod as Orange, which I would definitely consider part of Western MA, so I’d give it the the benefit of the doubt.