I am using SSIMVal in order to determine the similaritiy between images, what can I use to determine the cutoff point?

I know the closer to 1 the closer the images are in similarity but what exactly determines when close is not close enough? Like the value being .889 or .598? Or do I just say if I get a negative value or a value close to 0 it is not the same image or has significant differences that need to be accounted for?