Would not this be done via
You are correct however creating an invoice using it
Which looks OK
Costs are distributed in the appropriate account
and Taxes go to appropriate accounts
But the Tax summary report does not break the costs up.
Individual costs can be calculated from this report as the ratio is defined by the tax code definition.
I agree Manager would provide better functionality if a composite tax code consisted of ordinary tax codes. So the tax summary report would show the actual tax code data (IGST, CGST, SGST) and different composite tax codes could be created by the user to support different useful combinations. For more details on a general solution see Future Tax code structure ideas