I've updated my materia overmelding spreadsheet to remove a nonstandard Google Sheets function I was using, so now you can download it and work with it offline in the spreadsheet program of your choice! I've tested it with LibreOffice .ods format; presumably it works with Excel too. Google's export messes up the plot formatting, but that's not terribly important in the grand scheme of things.
Once again, if you are planning to do a bunch of overmelding, this sheet lets you estimate how many materia you'll need. To use it, you first plan out how many materia of each kind you'll have in each overmeld slot. Then you enter that data into the spreadsheet, along with how confident you want to be that all the melds will eventually succeed, and read off the number of materia you'll need to start with to have that confidence level.
Link to sheet: https://docs.google.com/spreadsheets/d/1h0YEDk9OFtkL3uV3zojEgcvu2bASnBYWiqJvn1o-_mI/edit?usp=sharing