In order to allow for the proper calculation of tax in both Manage and Sage 50, you need to map the Tax Code you have set up in Manage to the tax code you have set up in Sage 50.
The "Tax Code Xref" field of the Setup > Setup Tables > Tax Codes screen in Manage should be consistent with the "Sales Tax Code" in the Maintain > Sales Taxes screen in Sage 50. The "Agency Xref" field should be consistent with the "Sales Tax Agency ID" fields.
Please note, tax codes in Manage are done in decimal format. Sage is in percentage format. Example .04 in Manage is 4% in Sage. If you match the numbers exactly from Manage you would end up with .04% in Sage which would not map correctly.