Per thread buffers: What are they really?

Basically commenting on memory-usage

When computing maximum memory usage for a thread I have one source that says:

another that says:

yet another:

Regardless, all attest the importance of keeping per thread memory allocation reasonable; hoverver, there is no way to even generically estimate what it could/should be?

I’m not sure how it does it, but a tool I found tells you at the top what the max possible usage would be with the settings you have:

mysqltuner uses the first method. In full disclosure, that is the very reason why I sought a hard, fast answer. But there doesn’t seem to be one.