As radio telescopes become ever larger, and evermore capable, they can see ever further into the Universe. Together with my coauthors, quite literally some of the greatest radio astronomers in the world, we realised that several equations used in radio observations were just too inaccurate (if not wrong!) for the distant universe. This paper is our attempt to give a guidebook and rigorously derived, and checked, equations for this future era of radio observation with the Square Kilometre Array. You can also have fun with the online observation calculator to see what we mean. Fun story I first started doing these calculations with Martin and rest about 6 years ago. Happy to finally see this published!