@Mark Robson Welcome to Microsoft Q&A, Thank you for posting your query!
Typically we recommend a dedicated cache volume of 10 -20% of data size and then monitor cache hit ratio to fine tune. Towards the top end of that range for very hot data with more random access patterns and less than 10% for archive (for archive you do not expect to hit cache anyway). For sizing guidance on the hardware see Planning for an Azure File Sync deployment | Microsoft Learn but in the migration scenario ignore the initial sync memory guidance.
Hope this helps!
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