Upgrade Matrix
Ranking by supply lines
Recommended upgrade paths:
ExampleManufactory (number of trade routes) [FT] = Fast Travel proximity [MC) = Medium Commute <LC> = Long Commute Priority goes from top to bottom of the list for each region.
Coast of Africa:
Harufu (11) [FT] Port des Mines (7) [MC) Fond-de-la-Baie (7) <LC> Bandari (6) [FT] Jiwe (6) [MC)
East Indies:
Suny Capital (6) [FT] Damai (5) [FT] Laar (5) <LC>
Red Isle:
Gruérande (2) [FT] [Special case] Tanjona (6) [FT] Shijavu (5) [MC) Palisade Bay (4) [MC)
Special Case Gruérande is best for upgrading in early helm due to fast commute and close proximity to turn in. Due to this circumstance it is more advantagious and has been ranked higher than by the standard formula.
Credit to SgtShultz https://www.reddit.com/r/SkullAndBonesGame/comments/1b0vu0u/my_current_helm_priority_upgrade_list_for/