src/Model/Modifiers/Tax/FlatTax.php
public function value($incoming)
    {
        $this->Rate = self::config()->rate;
        //inclusive tax requires a different calculation
        return self::config()->exclusive
            ?
            $incoming * $this->Rate
            :
            $incoming - round($incoming / (1 + $this->Rate), Order::config()->rounding_precision);
    }
public function value($incoming)
    {
        $this->Rate = self::config()->rate;
        //inclusive tax requires a different calculation
        return self::config()->exclusive
            ?
            $incoming * $this->Rate
            :
            $incoming - round($incoming / (1 + $this->Rate), Order::config()->rounding_precision);
    }
Get the tax amount to charge on the order.