Hi,
I have created some code for calculating tax at item level. Here is the link for it. This method checks for the tax rate at item level first, and calculates the tax amount based on that. If the tax rate is not found at item level, it calculates the tax at invoice level. Have added some custom fields on that for state and gstin and HSN code on Sales Invoice. Though, the code is a little long, it works for me. Maybe, ERPNext might come up with some simpler code… Till then, you can check this out.
Regards
Uma