HexSys Grouped Quantity and Stock Control

Thank you, I will request a quote!

Hello, I am developing a site for a teashop which offers the products in packets of a certain weight, for example 50 g, 100 g, 150 g, 250 g, with different prices. I already created a system with child products, but the problem is the stock control. I don't want separated stocks for each child product, it should be like this: when a customer buys a packet of 150 g, the total quantity of the parent product in stock should be diminished with 150.
I read the documentation of the plugin HexSys Grouped Quantity Attribute For Virtuemart and I think it could be a solution, but it is not clear what happens with the stock level: if a person buys a product with grouped quantity of 250, is the stock level diminished with 250 or with 1?