Magento – Shopping cart price rule applied at Grand total (or subtotal after discount)

I followed the instructions in the following post:

https://stackoverflow.com/a/16808826

..which works on my Magento CE 1.7.0.2 install. You end up with an extra condition in your Shopping Cart Price Rules, "Subtotal with Discount" which will correctly return the current subtotal minus any existing discounts (perfect for free shipping rules!)

Hope this helps.

edit: As requested below, here is the code - you'd need to rewrite Mage_SalesRule_Model_Rule_Condition_Address and amend the class name etc accordingly:

class YourCompany_SalesRule_Model_Rule_Condition_Address extends Mage_SalesRule_Model_Rule_Condition_Address {

    /**
     * (non-PHPdoc)
     * @see Mage_SalesRule_Model_Rule_Condition_Address::loadAttributeOptions()
     */
    public function loadAttributeOptions()
    {
        parent::loadAttributeOptions();

        $attributes = $this->getAttributeOption();

        $attributes['base_subtotal_with_discount'] = Mage::helper('salesrule')->__('Subtotal with discount');

        $this->setAttributeOption($attributes);

        return $this;
    }


    /**
     * (non-PHPdoc)
     * @see Mage_SalesRule_Model_Rule_Condition_Address::getInputType()
     */
    public function getInputType()
    {
        if ($this->getAttribute() == 'base_subtotal_with_discount')
            return 'numeric';

        return parent::getInputType();
    }


    /**
     * Add field "base_subtotal_with_discount" to address.
     * It is need to validate the "base_subtotal_with_discount" attribute
     * 
     * @see Mage_SalesRule_Model_Rule_Condition_Address::validate()
     */
    public function validate(Varien_Object $address)
    {
        $address->setBaseSubtotalWithDiscount($address->getBaseSubtotal() + $address->getDiscountAmount());

        return parent::validate($address);
    }
}

I think you can achieve this by setting priorities to your rules.
So your 30% discount rule should be processed last.
This way it will take into consideration the subtotal with the discount from other applied rules.
So set for this rule an outrageous priority number to be sure nothing is processed after it.
Something like 99999.