How-to return product Quantity for validation

I would really appreciate some help on this one.

All I need to do is to populate a list field with a qty collected in the first step.
Here is the code:

// Populate list with selected quantity
add_filter("gform_pre_render_1", "pre_populate_row_list");
function pre_populate_row_list($form){

	$field_id = 15; // Member list
	$product_qty = rgpost('input_30');
	$list = rgpost('input_15');

	$current_page = GFFormDisplay::get_current_page($form["id"]);

	//if($current_page == 2){
		foreach($form['fields'] as &$field){
			if($field['id'] != $field_id)
				continue;	

			// Dynamic Population doesn't work :\
			$array = array_fill(0, $product_qty ,'');
			$field['choices'] = array_fill(0, 2, $array);
			//$field['label'] = implode(',',$field['choices']);
		}
	//}
	return $form;
}

Thanks. Thats how I did it finaly.

add_filter("gform_field_validation_1_15", "validate_list_field", 10, 4);
function validate_list_field($result, $value, $form, $field){

    $product_qty = rgpost('input_33');

    // Test if equal quantity
    if( count($current_emails) != $product_qty ){
        $result["is_valid"] = false;
        $result["message"] .= sprintf(__('<br/>Vous devez inscrire (%d) personnes dans votre équipe.','nsec'), $product_qty);
    }

    // always return result
    return $result;
}