Hi,
In your case, I would create a custom field of the table "item" of the type "dropdown" for the selection of the aditional 1/2 1/1/4 or 3/4 yard on the product page, and then a small plugin to override the price of the product in the cart based on the quantity and what's entered in that custom field.
This would require the Business edition, and the development of that plugin, which can be based on the width*length price override plugin that we offer in the "How to sell products by Width and Length ?" section of our FAQ:
www.hikashop.com/support/documentation/106-faq.html#cart
If you need a developer to work on that for you, you can contact our partners:
www.hikashop.com/home/our-partners.html