Substract option quantity from main stock

Hi guys i have a question. How can i substract product option quantity value from main stock. I mean i set a checkbox option for a product and i set quantity value to 5 and set substract stock to NO. What i want it's if i buy that product, the main product stock to be minus that value i mean 5 not 1. Can anyone know a solution? Thanks in advance!

Look at the OpenStock extension from Welford Media