Calculate quantity (not price) with plugin

-- HikaShop version -- : 2.5.0
-- Joomla version -- : 3.4.3
-- PHP version -- : 5.4.40
-- Browser(s) name and version -- : Firefox 39

Hi,

I'm currently working on a site where products (floors) are priced per m² (square meter) but are sold per package.
The contents per package is variable per product.

I added a custom field (inhoud_pak) which contains the number of m² in the package.

Customers need to be able to enter the lengt and width of the floor. Based on that length and width I want the quantity to change.

I dowloaded and installed the custom_quantity_width_length plugin and changed the calculation to

if(!empty($product->inhoud_pak)) $quantity = ceil(($quantity*$product->length*$product->width)/$product->inhoud_pak);

Hi Nicolas,

Thank you for your answer. Changing $product->cart_product_quantity works, but changing the quantity in the cart still returns wrong values:

After

if(!empty($product->inhoud_pak)) $quantity = ceil(($quantity*$product->length*$product->width)/$product->inhoud_pak);

$product->cart_product_quantity = $quantity;

where is plug in
for reader , have a look also to this post .
what is strange is that this post speak about hikashop custom quantity plugin and when i go to plugin manager to search it i don't have any ?
Where is locate the plug in ?
which is the name of the php file to edit ? ( if unique then i could search it's location on my OS filesystem )