Thanks but just a few things...
1) Each tier has 4 grades for a total of 16 in all. Tier 4 stats give 16 17 18 20. (no +19) This is from the Dats.
2) It would be nice to include the level of the gear converted in your spreadsheet as that affects the probabilities.

In my own findings, it appears that all gears of the same equipment slot give the same types of materia. So I agree with your organization of the output based on slots.
E.g. Crafting tools, both main and offhand only gives crafting related materia. Even though you can use stat materias on mainhand tools, you don't get them from crafting tools.