In order to be able to model even the most complex taxes, GM offers a formula builder with which you can manually input a formula by which the taxes are calculated.
To use the tax formula builder you have to set the reference of the taxes to “Custom”. Thereafter, you can chose the aggregation period to which the formulas are referenced to. As example, if you use sales from last period, the period will be defined as the aggregation period. You can use the same functions as in Excel, including Min/Max and average. There is a tooltip next to “Formula” which explains what variables can be used and how to use them.