Using PAR2 to generate parity information to use up the remaining space on a CD or DVD, to increase chances of recovering from minor discrot or scratches
A DVD has 2295104 2048-byte sectors. Expect to use around 2-4MB for the ISO/Joliet/Rock Ridge filesystem structures. More, if you have a ton of small files.
Usable data space is, roughly, 4,696,178,688 bytes.
I choose a parchive block size of 262144 bytes (256kB) to keep the block count low, and to keep the block size a multiple of a DVD or CD ECC group size (32kB) to minimize wasted space.