When using the Structure Sensor with Canvas, you should try to be anywhere from .5 - 1m away from the surface you are trying to scan. This will ensure the best accuracy possible.
I'd also like to note that the precision of the Structure Sensor is going to be different from the accuracy that the Canvas App can achieve.
We are still collecting data and running case studies but, from what we have seen working with professionals in the field is that most measurements are within 1-2% when verified against a manual tape measure or existing blueprints.
Usually, accuracy is improved as the user gains experience with using the Structure Sensor. You can find tips on how best to use the Structure Sensor with Canvas here: https://canvas.io/support/do-you-have-any-tips-getting-great-scan