Income Tax on Taxable Turnover

That's the taxable base taken from the GST returns.

For income tax the declared income (before expenses) cannot typically be lower than that. In fact, it will usually be higher.

E.g.: 19 lakh business / professional income (less related expenses) + other incomes like interest, dividend, capital gains etc.

Yes,he is liable to consider the same amount for income tax turnover calculation also less business expenses.