That scratch is not really much of anything to be concerned about. I have a lens with multiple scratches on the front AND rear elements, deep enough to catch a fingernail on, and you wouldn't know it to see the images it makes. I gave it a torture test- photographed in the rain, at night, with neon lights in the frame, and no flare that I could detect. I've even seen some lenses that have actual cracks the whole way through the element that still make great images (although I would never point them at a light source).