Calculate Field with a Lookup Additional Column

Would like to ask some suggestion if i have 2 lists (Vendor List & Order List). Where Vendor List is contain with name & discount fields

And i create new list "Order List" with field Items, Price, Vendor (lookup from vendor list)

When i create a lookup field to the "Vendor List", i add additional column (Discount) too.

Then after i add new list, then the record will shown as below. What i would like to ask is, how to do calculation between price in the each item, with the discount additional lookup field? I look some reference about workflow in SharePoint Designer, as a newbie i still cannot have it :(.

Looking some input for all you guys. Big thanks...