Pam,
The first thing that came to mind about the binding not laying flat is if it is the wrong type binding. That is, does the piece you are binding hqave four straight edges like a bed quilt? Or does it have rounded corners, or maybe is an oval?
If it has four straight sides with 90-degree corners, traditional binding cut on the straight of grain should lay flat. The only thing I can think of is that either the fabric grain was slightly skewed before you cut it, or you have inadvertently stretched the binding as you attached it and skewed it slightly.
If your table runner has curved sides, you definitely need to be using bias-cut binding, which means you have cut the binding at a 45-degree angle to the true grain.
The rate of speed that fabric goes through at the mills it is easy for it to get twisted or skewed, but is easily rectified. You can straigthen the grain of a fabric very easily, but if your length of fabric is very long, you will need a partner.
Each of you take one corner, diagonally opposite the other. Pull on the fabric two or three times. Switch to the other set of diagonal corners and repeat. Repeat the whole process once more, this should re-align the warp and weft threads so they are perpendicular to one another.
IMO, the width of the binding shouldn't be a factor to the problem you are describing. Although, if it is very wide so that easing it in is difficult, I can see that being a problem.
This is an issue of personal preference, I suppose, but I was taught that the binding and the borders (if there are any) function the same way that the mat and the frame do on a picture. They are design elements to a certain degree, but their primary function is to define the boundries and help focus the viewe's eye on the center. As such, they need to be in proportion to the center elements.
I think a binding on a table runner that is wider than the binding on a bed quilt might look odd, but that is just my personal preference. And you can't make it too much smaller, because it just gets too awkward to work with.
Before you go to all the trouble of figuring out new deminsions, etc., check to see if the binding you have already cut isn't slightly skewed.
Welcome to the forum, Pam. We are very happy you have sought out our counsel and would love to hear of your progress. If you read many of my posts, you will see that I do not believe in the quilt police and I do not believe in rules, only guidelines.
My approach to quilting is pretty much the same philososphy my parents used in raising me (my boys are pretty much grown, so my quilts are my babies now.)Within the boudries of what was age appropriate, I was pretty much allowed to choose what I wanted to do. With that freedom came the responsibilty to make an informed decision. I could use any source for my reasearch and if I made a bad decision, I lived with the consequences.
I have been quilting and sewing for 50 years. My mother is nearly 90 and still quilts. If she is any indication, I've got another 40 years. So here are the only two rules I quilt by.
1) There is no wrong way and no right way; there is only your way and my way. What works for one person in one instance may not work for someone else, but it is always good to seek opinions and make an informed decision.
2) Seek the simpliest solution that will satisfy you. There is often more than one remedy to a problem. The right one is the one that keeps your satisfaction and your aggrevation in balance.
Have a great day, and please keep us posted on your progress with this porject. You might want to pop up to the introductions thread and share just a bit about yourself. We love making new friends, and you will find the folks here are the friendliest on earth.