Territories are valued, not drawn. Without account valuations there is no project — you cannot balance what you haven't priced. Value every account, normalise before you balance, then cut in a defined order and never cut below the metro level.
Design three steps ahead of current headcount so the map survives the next two hires instead of being redrawn each time. And treat it as cross-functional: territory design touches marketing, CS and finance, not just sales, because it changes coverage, routing and quota simultaneously.
Every claim above comes from work LeanScale published. These are the sources.