Different Quantity Discounts for customers from Group Pricing?

[thenax]

To give customers who are in a certain customer group pricing group (customers_group_pricing = 1)
only half (0.5) the regular quantity discount, I modified to following files:

\includes\modules\products_quantity_discounts.php
This file will display the quantity discount at the product information page.
Just above the code

$display_price = zen_get_products_base_price($products_id_current);
$display_specials_price = zen_get_products_special_price($products_id_current, true);
$disc_cnt = 1;
$quantityDiscounts = array();
$columnCount = 0;

I included:

    //modification by pe7er
    //check if current customer is part of a certain (customers_group_pricing) group pricing (zen_group_pricing -> 1) category
    $gpcheck = $db->Execute("select count(*) as count from " . TABLE_CUSTOMERS . " where customers_id = '".$_SESSION['customer_id']."' AND customers_group_pricing = '1'; ");
    if ($gpcheck->fields['count'] >0){
      $distri_discount = 0.5; // the customers in this price group get 50% of the quantity discount
    }else{
      $distri_discount = 1; // other customers will receive the full quantity discount
    }// end modification.

and I changed (about 6 lines)

= $display_price - (($products_discounts_query->fields['discount_price'] *$distri_discount)/100);

into

= $display_price - ($display_price * (($products_discounts_query->fields['discount_price'] *$distri_discount)/100));

\shop\includes\functions\functions_prices.php
This file calculates the quantity prices.

Just AFTER:

$display_specials_price = zen_get_products_special_price($product_id, true);

I added:

    //modification by pe7er
    //check if current customer is part of a certain (customers_group_pricing) group pricing (zen_group_pricing -> 1) category
    $gpcheck = $db->Execute("select count(*) as count from " . TABLE_CUSTOMERS . " where customers_id = '".$_SESSION['customer_id']."' AND customers_group_pricing = '1'; ");
    if ($gpcheck->fields['count'] >0){
      $distri_discount = 0.5;
    }else{
      $distri_discount = 1;
    }       // end modification.

and I changed (about 6 lines):

($products_discounts_query->fields['discount_price'])/100);

into:

(($distri_discount * $products_discounts_query->fields['discount_price'])/100));

Hi..perhaps you can help me.
I have only one price group (1) that has 5% discount
how can I do to apply 5% if total order is max 200 Dollars, +50% (7.5%) if total order is max 600 Dollars and +100% (10%) if total orders is more than 600 dollars (only to group price 1)?

thank you very much