The depth accuracy of the 400 Series cameras is less than one percent of the distance from the object. So if the camera is 1 meter from the object, the expected accuracy is between 2.5 mm to 5 mm.
There are numerous environmental factors that can have an impact on accuracy. Also, in regard to technical factors, the 'RMS error' in the depth readings will increase as the distance from the object increases. It only becomes really noticeable at around the 3 m point though, so you should not experience much accuracy drift if using a distance between 100 and 1000 mm.
Pages 13 and 14 of Intel's excellent illustrated tuning guide provide information and charts about the camera's RMS error factor.
https://realsense.intel.com/wp-content/uploads/sites/63/BKM-For-Tuning-D435-and-D415-Cameras-Webinar_Rev3.pdf
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Please also note that the RealSense forum has moved location, so future questions should be posted at the new forum linked to below. Thanks!
https://support.intelrealsense.com/hc/en-us/community/topics