I'd like to know if there's some rule (or formula) I can apply to find out how much of disk space will be used by the filesystem in an ext4 partition. for example, in a partition of 100 GB, how much can I actually use? does it depend on other parameters like inode size, etc?
I don't know if it applies to you, but just in case: most of the time the huge majority of the apparent overhead of ext2/3/4 will be due to the default significant amount of reserved blocks for root. Make sure you specify
-m 0 (or 1) to mkfs.ext4 when creating the filesystem or to
tune2fs if adjusting it afterwards. The default is 5, which stands for 5% of total blocks and can be excessive in most cases.