Ministers are happy if the specific voting groups that they get their support from are kept happy. The amount of capital they produce also declines over time, if the ‘political honeymoon’ option is turned on.
The equation you reference is linked to a hidden value called Tax Evasion. Basically what this line says is that you take 20% plus 80% multiplied by the current level of GDP, and then you add the value of 100% minus 20% of the current value of tax evasion, and that final percentage is sued to multiply by the expected tax income.
In other words, tax evasion can reduce the income from sales tax by up to 20%, and the income from the tax is also scaled by GDP, although there is a minimum of 20% (even when GDP is rock bottom),
Hope that helps,