Hi,
There is no system to have a coefficient on the quantity of the variants of products.
So that would require custom plugin to calculate that if you decide to keep it like that.
Alternatively, you could have your quantity in grams. So for example, you would have 50000 of stock if you have 50kg of that product in stock.
And then, instead of using variants, you could instead add prices with "minimum quantity" for each step you want for the selection and then set the "quantity input method" option of the product to "show select price".
So if you have prices like that for the product:
min qty | price
500 | x
750 | x*1.5
1000 | x*2
...etc...
You would get a quantity input as a dropdown with the quantities 500, 750, 1000, etc available in it.
That way, you would get a unified stock control for all the choices, but you'll loose a bit on display since you can choose what will be displayed in the dropdown for each choice. It will just be the numbers (unless you customize the display of course).