Overcooking your salmon will cause it to dry out! And, sad news: it doesn't taste good. You can use a meat thermometer if you want to get very precise with your temperature - but I usually don't use one and I just go off of how it looks and feels. I'll often flake apart one piece to see how it looks before deciding if it's done or if it needs another minute or two in the oven, even though it messes up the presentation slightly. I find that regardless of size of filets or oven variations, somewhere between 10-15 minutes is usually the right amount of time for the doneness that I like.