The Meaning of Life, the Universe, and Everything.
Even though the game is make of cubes, distances are spheres, the easiest way to represent how it works, go into creative, pillar up into the sky so you have room to work. Make any size circle as if your placing it on the ground, then place a second circle exactly the same size going through the middle of that circle so half would be overground and half under. Then do the same for the diagonals. You'll then hace a skeleton sphere and can see how the game will calculate distance from you, any spawn calculations for farms, light generation etc. All you'll need to do it freehand is know your centre point of the sphere and the radius to the edge in blocks.
It all depends how you want to use that infor, almost every type of perimeter should be spherical but they're always square because its the easiest and quickest to work and and save time so its down to time and preference really.