I am also a 7.5/8 and all my flats are a 38.5 or 39... I wish I had better news for you. If they don't fit, do you think you could resell them on eBay at what you got them for?? Or will you be able to return them? It's always worth a shot, but I agree with the other posters that satin is less forgiving.
I needed to size up 1 whole size from my normal US size. I tried my normal size and they were uncomfortably tight, cutting into my heel.ush: