I've attached an interactive guide that helps display the relationship between map size, purchase points, and battalion frontage.
Frontage reference:
Guide previews:
Here's the same guide but the average battalion cost has been raised to 5000 to model armoured or more modern battalions:
Here the guide has the average battalion cost raised to 6000, and the frontages have been changed to 6000 and 3000, for modern mobile forces:
There's notes and explanations in the downloadable spreadsheet.
see attached...
Cross