While you might be able to calulate the actual amount of metadata required, you can't turn on GNA with raw SSD capacity less than 1.5% of raw HDD. For the foreseeable future, it is safest to follow the guideline of 2% capacity (though you could go as low as 1.5% in current OneFS versions), and 20% of nodes.
I have been trying to track down a way to determine how much SSD needs to be added to an existing cluster for metadata acceleration based on the amount of data that exists on the cluster already. I am aware of the recommendations in regard to percentages, but determining how much space on the SSD will be utilized is what I'm looking for. Any thoughts?
Sizing: How much SSD do I need to add to a cluster for metadata acceleration?