New tax rate requires reduced base amount?

Hi all

I need to create a new tax code which requires applying 5% Federal Sales Tax on Base Amount and 5% Provincial Sales Tax of 30% of the Base Amount, which means the vendor invoice will have both taxes but Provincial Sales Tax should be calculated @ 5% of 30% of the Base Amount (not the full base amount).

This is the first time I have to create such a tax code and I understand that I will have to create a line in my calculation procedure for this Reduced Base Amount and I can calculate 5% Provincial Sales Tax using that line. However, I am struggling to understand how to get 30% of base amount on this reduced base amount line.

I would appreciate if someone could guide if this is the correct method and if so, how to pull 30% base amount on this line and if not, what is the correct step by step procedure.

Thanks for your time.

Moazzam