If you have not already done so, you should review the Industry Steering Committee on Wellbore Survey Accuracy (www.iscwsa.net
) documents found under "docs and publications" tab and then "Collision Avoidance Sub-Committee". Especially useful is the document "Current Common Practice in Collision Avoidance Calculations". http://www.iscwsa.net/download/171ca15c-a5a0-11e6-93ba-a765019e26b7/
I assume you are referring to page 172 in the ISCWSA Education Committee's Joint Industry Project e-book "Introuduction to Wellbore Positioning" V06.02.17: https://www.uhi.ac.uk/en/wellbore-positioning-download/
The error at each station is the error at the previous station added (or Root-Sum-Squared, depending upon propagation mode) to the error between the previous and current station. Professional wellplanning software such as Compass, Well Architect, HawkEye3D, KellyDown, and others calculate these errors for use in anticollision calculations. These are not trivial calculations. The ISCWSA error model committee provides some reference wellpaths with the values calculated to validate the results of different software. http://www.iscwsa.net/download/71dd98c7-3c98-11e7-8b8c-dfeb929446d3/
This is a Microsoft Word document with an embededded Excel Workbook with the wellpaths, 1 sigma errors in HLA (Highside, Lateral, Alonghole) form, and calculated clearance factors.