Hi @Nivu and welcome to the Quick Sight community!
So do you mean to say that ‘Tax’ should start from the base? So is ‘tax’ a value, like ‘Sales’ is, for your ‘Line Item’ field or is it a value within another field (along with ‘other expenses’, advertising, etc..)?
To alter a waterfall, you would need to add a field to breakdown, as you can see below in my example:
Before
After
Additionally, it may be more beneficial to assist further if you were able to put together a sample version of your analysis with anonymized data into the Quick Sight Arena view: