My quilts happen in a variety of ways. It can be a pattern that I particularly want to make and need to purchase fabric for; or it can be fabric that I love and need to search for the right pattern to use. I don't usually use pre cuts, although I have purchased them because I love the fabric selections. They remain in my stash. The only quilts I can recall actually making with pre-cuts are a couple of jelly-roll race type quilts.
I don't usually have much trouble with conversions or calculating fabric requirements.