Withholdings in income tax for non-residents without a permanent establishment
Withholdings applicable to non-residents and tax return forms for withholding agents
General rule for determining the withholding tax base
Generally speaking, the amount withheld should be a quantity equivalent to the tax liability that derives from the tax. However, the withholder will not need to take into account the deductible expenses or deductions (which are indeed applicable when calculating the tax).