Shard failure rate source?

[cameronelliott]

While not strictly needed to use this cool program,
it would really add some value if the source of the shard failure rate for both
the examples of invoking this calculator.

The two examples are:

python durability.py 4 2 0.10 1
python durability.py 17 3 0.00405 6.5

Has anyone at Backblaze written about determining an estimate for the annual shard failure rate?

It would be fascinating to hear how these numbers were derived.
Any sources or blog posts would be cool too.
(I understand the annual drive failure rates published by BB are around 1% or 1.5% per annum,
but turning that in the annual shard failure rate is something I'd love to hear about)

Thanks.