Hi,
In a nutshell, the recommended value should be enough to fit the "window" for your live streams available to users.
For example, if you have 100 HLS streams and there are ~10 chunks available to users in every stream playlist with average chunk size is about 1 MB, then you need at least 1MBx10x100=1000MB to fit them all in cache. This is very rough estimation, that can vary depending on a workload, and this is not all the memory that will be used by Nimble, it will allocate some additional memory to server every active user session. I'd recommend you to start from at least 1.5GB of RAM cache to have some reserve, and monitor the cache usage for some time during the high workload hours.