Marathe & Steeply: the Convexity within the distance of spit
But also the city Boston U.S.A. has logic. Your closest city to the Convexity. Closest therefore to Québec. Within as you say the distance of spit.
Context
The reconfigured map made intimate and menacing: Boston is the A.F.R.'s logical Ops center because it sits "within the distance of spit" of the Convexity, hence of Québec. The gift-territory that was supposed to push the enemy away has instead put the border a spit's throw from a U.S. metropolis — the wall creates the very proximity it was meant to prevent, which is the whole geographic irony of Reconfiguration.