You'll want to start calibrating an hour or two before your seed comes around, depending on how long your wait time is. It helps since it gives you a feel for when you should press A, and once your seed comes you'll be ready to go. When I was getting Groudon, I failed the first time, but luckily, I got another chance half an hour later. I don't know what else to tell you, other than good luck.
Edit: Oh no! You already missed your frame. Sorry about that.