Yep, that sounds about right. I don't think you did anything wrong. I have made several jelly roll races. They do at some point get longer than I want and narrower than I want. That's when I have gone one step further than I should have. Now what I do, when it gets to a suitable width. I stop. Then I add strips (extra ones) to the top and the bottom a row at a time till I get the length I want. As long as the same color strip is added to each side, it isn't noticeable that strips have been added and it all blends in.