How to setup Tax Regime Configuration Options one OU with mutiple legal entity

Hi Guys

   I have one Asia OU and One Primary ledger with multiple legal entity.   These legal entity present for different county like Malaysia, Philippine, Thailand , Vietnam ect in Asia Pacific.

Because the office in those country are very small, so we have consolidate all the office in one OU(Asia OU).

Now come to my question , for each country will have one Tax Regime (Country Level).  And in the Tax Regime configuration Option normally need to assign the OU or Legal entity here, so the Tax Engine able to associate the supplier with Tax Regime. So should assign the OU or the legal entity here ?