Don't worry you did not make a mistake if you cut it like that. Like Bella said, the half inch is taken up by 1/4 inch seam when you attach it to the quilt and 1/4 inch seam when you attach the binding.
Listen to me, I am a new quilter too and here I am giving you advice. The people here are great and very helpful.
I find that you learn with each one you do. I remember when I made my first quilt and the triangles on some of the blocks did not match perfectly when I sewed them together. I wanted to rip them all out but did not have the time because it was a Christmas present for my son and his family. My sister told me, hey those tiny mistakes are what make it more precious and she is right. I have learned to lighten up. I still try and match my blocks as best as I can but I try not to stress if I cannot do it. I do not plan on showing them, I make them for the pleasure it gives me and for the people I love.
Listen, now that you know the border will be ok, go buy some more fabric, you never have enough!! LOL :lol:
Don't forget the pictures when you are done.