Incorrect tax with Group Discount

[Dave224]

I'm having trouble with tax calculations for different products with group discounts. The store has taxable and non-taxable products as well as products that are discountable and non-discountable for group members. Thus I have four conditions to handle in ot_group_pricing.php: (1) taxable and discountable, (2) taxable and non-discountable, (3) non-taxable and discountable, and (4) non-taxable and non-discountable. I've modified ot_group_pricing.php to calculate the totals for each of the four conditions plus calculate total taxable and total non-taxable. The correct discount amount is calculated. A ratio used in tax calculations is defined as:

$ratio = $od_amount['total']/$order_total;

in line 96 of the unmodified ot_group_pricing zc157c code. Later, $ratio is used in the following code:

case 'Standard':
          if ($od_amount['total'] >= $order_total) {
            $ratio = 1;
          }
          $adjustedTax = $orderTotalTax * $ratio;
          if ($order->info['tax'] == 0) return $od_amount;
          $ratioTax = ($orderTotalTax != 0 ) ? $adjustedTax/$orderTotalTax : 0;
          $tax_deduct = 0;
          foreach ($taxGroups as $key=>$value) {
            $od_amount['tax_groups'][$key] = $value * $ratioTax;
            $tax_deduct += $od_amount['tax_groups'][$key];
          }
          $od_amount['tax'] = $tax_deduct;
        break;

I think the ratio calculation needs to be modified, but I'm stuck. In the calculation of $ratio, should $order_total be replaced by the order discountable taxable total, the order discountable total, or something else? What exactly is $ratio supposed to represent? Any changes needed in the standard case block? The tax calculated is incorrect when mixed discountable and non-discountable products exist in the order.

Dave
zc157c, php7.3, only one tax - state sales tax