Provisional Tax - how to work it out?

New to provisional tax. We are working with an accountant but info is not forth-coming. Is the taxable amount worked out on:
Income less
Deductions / Expenses
= Taxable Income (i. E. We are taxed the percentage in that tax bracket)

i. E. The provisional tax amount is not just worked on your income?

Yes, the tax payable will be worked out on your taxable income: Income less business related expenses.

Please see further details around Provisional Tax at the link below.
What is Provisional Tax? How and when?