LVM Snapshot Storage Requirements

Unfortunately, the storage requirements are not quite that straight forward:

COW snapshots are created when a size is specified. The size is allocated from space in the VG, and is the amount of space that can be used for saving COW blocks as writes occur to the origin or snapshot. The size chosen should depend upon the amount of writes that are expected; often 20% of the origin LV is enough. If COW space runs low, it can be extended with extend (shrinking is also allowed with lvreduce.) A small amount of the COW snapshot LV size is used to track COW block locations, so the full size is not available for COW data blocks. Use lvs to check how much space is used, and see --monitor to to automatically extend the size to avoid running out of space. - man lvcreate

Note how the man page doesn't mention any particulars:

...often 20% of the origin LV is enough...A small amount of the COW snapshot LV size is used...

The conclusion I draw from this is that there's no specific way to calculate exactly what size the snapshot LV needs to be to ensure it never runs out of space.

Essentially, since the snapshot LV stores the original blocks prior to being changed, in order for it to never run out of space it would have to be large enough to hold the origin LV plus a little bit more so it can track the COW block locations.