As understanding of the Crystal Sphere Techniques increased and the details of the individual spheres was differentiated, the concept of the crystal sphere distance was established. Based on the abstract spherical units (su), spheres that have a short distance are easier to combine together whereas spheres that were further apart are more difficult and require more energy.
When it was originally developed by Gevilin, a spherical unit was set so a distance of 1 su was the energy and effort to combine fire (a reactional) and water (an elemental) magic to create steam. This was considered one of the easiest spells to cast for a beginner. At the far end, the effort to combine water and folding magic to transport a quantity of water a hundred leagues was considered teh baseline for one billion units (1 Gsu).