That's simple concept here, just one line in Galvin :
The table has one entry for each disk block and is indexed by block number.
now go step by step:
Step-1: Find no. of disk blocks as total size by disk size block
Step-2: Answer in step1 is nothing but no. of entries in FAT. And as you can see each entry is of 4 bytes so we can easily calculate size of FAT.
Step-3: Now, FAT is also stored on disk as line by Galvin :
A section of disk at the beginning of each volume is set aside to contain the table.
so simply subtract this with total size of disk.
Now, the answer you get in step-3 is amount left after storing FAT, that we can use to store file. :)