computed field with product variation and content type
To clarify from the comment thread:


This is a question that is tough to answer on SE because it's just something you'll have to feel out. If you're doing 1:1 content types to product displays, it doesn't really matter where you put it (though it'd probably be easier to do it on the CT) in general I tend to put more on PV because it's data related to the product. If you're doing 1:N then you'll definitely want to put it on the PV and if you're doing M:N then you're going to have to do some customization.
You can put the computed field on the PV but you'll need something like Entity Backreference so that the PV has a link back to the content type so you can load it and grab it's fields.