It varies depending on where the moon is in the sky. If the Moon, or Sun, is low in the sky then the ISS will be further away, maybe 1000-2000 kms away, and the path width will be larger. Ideally you want to wait for a transit when the Moon is high because at zenith the ISS is only about 400kms away and will be much bigger in your photo or video. Personally I wouldn't be more than 500m from the centre line as these transits are so short anyway that you want the ISS to pass through the centre of the disk and maximise the transit time.
Along with Calsky I also use this site, and mainly his Android app, to get my predictions:
http://pictures.ed-morana.com/ISSTransits/predictions/
This site/app also shows the path width.
When you find a transit you're going to try for make sure you recheck Calsky regularly as the prediction will be refined and updated with any new ISS orbit changes.
It's a great challenge, good luck!