I'm making a baby quilt right now & my finished blocks are 7 1/2" square. I've put on a 1 1/4" border (finished size) around for the first border & then a 2" border (finished size) around for a second border. I like the way it looks. I never thought that there might be some sort of formula before your post here. I just go by what I think looks good. Sometimes I need to enlarge a quilt so I will do it with borders.