I add a little extra for mistakes on my part and on the part of the pattern maker. I do this on all the quilts I buy fabric for and try to on any kits I buy. I sometimes change my mind about a color and sometimes I will buy an alternative color for the quilt. It is not like you can have to much fabric.
I made the mistake when I started quilting that I did not buy yardage of fabric I really like and I am really not much of a scrap quilter unless it is a sampler quilt.
Oh well Live and learn.
I am trying to build a stash but keep using all my abric up!!
tricia