How to estimate / calculate the FILEX memory pool size if ExFAT feature is enabled?

[HTD]

Here's my FILEX configuration:

It works with 128kB pool size, but if I set the size to 64kB, it doesn't, I get hard faults while trying to write a file on ExFAT formatted media (USB disk). It works with even smaller pool on FAT32. How should I estimate / calculate the pool size properly? Now I just guessed the working value.