You can try to using AWA (Automated Working Set Analyser) to sizing how many cache capacity you need.
Max Projected Cache Size
The size at which the SSD cache would hold every eligible data block that was requested from disk during the AWA run. Note that this does not guarantee a hit for all future I/O operations, because they might request data that is not in the cache. However, if the workload during the AWA run was a typical one, and if your budget allows for it, this would be an ideal size for your Flash Pool cache.