When creating a Payroll Tax Budget Formula, this will need to be constructed in two parts.

Add a Metric called Payroll over Threshold to calculate the addition of the values in the GL Accounts less the tax free amount:

Add a Formula to the Metric Payroll over Threshold:

(Salary/Wages + Superannuation + Allowances) - ($value for Payroll tax free amount)

If the Payroll tax free amount is not a static value, you can create a separate metric to calculate that as well.

Then in the Payroll Tax Expense GL Account add a Formula:

(Metric Payroll over Threshold) * (percentage rate payable as a decimal eg 0.0495)

to get the amount payable each month.