WooCommerce – Fixed quantity of a product

I am trying to accomplish a thing on WooCommerce that I can not find any information about. It is like the following scenario:

I sell screws and have 1500 in stock. I would like the customer to choose between buying 10, 30, 50 and 100 screws in a drop-down list. The price for each screw is lower when you buy 100 compared to 10, 30 etc.

This works just the way I want by using product attributes and product variations. BUT the stock is not effected the way I want since the system think that it is one package of 100 screws and therefore reduce the stock by 1.

I am thinking of modifying the class-wc-product-variation.php file that contains the following code:

function reduce_stock( $by=1 ) {
    global $woocommerce;
    if ( $this->variation_has_stock ) {
        if ( $this->managing_stock() ) {
            $this->stock        = $this->stock - $by;
            $this->total_stock  = $this->total_stock - $by;
            update_post_meta( $this->variation_id, '_stock', $this->stock );
            $woocommerce->clear_product_transients( $this->id ); // Clear transient
            // Check parents out of stock attribute
            if ( ! $this->is_in_stock() ) {
                // Check parent
                $parent_product = new WC_Product( $this->id );
                // Only continue if the parent has backorders off
                if ( ! $parent_product->backorders_allowed() && $parent_product->get_total_stock() <= 0 ) {
                    update_post_meta( $this->id, '_stock_status', 'outofstock' );
                }
            }
            return $this->stock;
        }
    } else {
        return parent::reduce_stock( $by );
    }
}

I am trying to set $by to the name of the product variation (10, 30, 50, 100) but I am not that good at PHP. I have tried to collect some relevant code (at least what I think) and putted it together to the following function:

function new_reduce_by () {
    //Get the name of the variation
    $this->variation_data[$name] = $attrvalue[0];
    //Extract only the numbers (some attributes may end with "grams" etc.)
    preg_match_all('!d+!', $attrvalue[0], $modified_attrvalue);
    //Convert the array to a variable
    $custom_reduce_value = implode(' ', $modified_attrvalue[0]);
    //Return the name (value) of the variation
    return $custom_reduce_value;
}

I have tried to call this function in different ways but without success, I guess the code above is not correct.

Should the code above work? If so, how do I call the function in the best way? I know that you cant call a function by changing $by=1, you have to pass a value or variable.

Or should I solve this some other way?

Many thanks!