Ah, yes... You're right, Jason. I am confusing database file free space with free space on disk. I feel pretty dumb now!
Thanks, Jeff! It looks like I, at first, was trying to keep the 15% "rule" for the database files, but since the database is so large 15% is a huge amount of space. So I stopped maintaining the 15% rule (within the database) and so I was curious if I was mess something up. Now that Jason blew my mind with his database file free space vs free space on disk, I'm all straightened out.
I would absolutely LOVE it if I could move old data out, but everyone else and their mammas want to keep that old data all in a single database. I do have the largest tables/indexes partitioned.
I will look into the page fullness for each index. I didn't think it was recommended to perform index reorgs/rebuilds anymore. Only to keep up with stats.