You should define the default intercompany balancing rule at the legal entity level, because each ledger has one legal entity assigned to it and each legal entity is associated with one balancing segment value. This way, you can specify the intercompany receivables and payables accounts for each legal entity and ensure that the journals are balanced by legal entity or primary balancing segment values. A ledger-level rule would apply to all legal entities in the ledger, which may not be appropriate if they have different intercompany accounts. A primary balancing segment rule would apply to all ledgers that share the same chart of accounts, which may not be desirable if they have different intercompany rules. A chart of accounts rule would apply to all ledgers and legal entities that use the same chart of accounts, which may not be feasible if they have different intercompany segments or accounts. References:

Intercompany Balancing Rules, Section: Define Intercompany Balancing Rules

Overview of Intercompany Balancing Rules, Section: Define Intercompany Balancing Rules

Troubleshooting Guide For Intercompany Balancing, Section: 1. Journals and subledger accounting entries are not being balanced for intercompany activity

Implement General Ledger, Section: Intercompany Balancing