There's a calibrate section on the tool bar that pops up when you connect to your pc. It's best if you run a route you know the exact length of then calibrate it to that when you go to sync. I found a bit of info about it on the Nike website. Good luck with it.
Hi Wyn. I used Nike as well. I had a bit of trouble to begin with but when I calibrated it a couple of times via the Nike site it seemed fine. Nike are also very good at resolving problems when you contact them via email or twitter. Hope it helps