Currently the description of this property in the reference guide is:
The description suggests that this is the maximum amount of memory that is allowed to be utilised for this entry cache as a % of JVM heap.
It seems that the actual use of this parameter is the maximum amount of memory currently in use by the whole JVM, before the Entry Cache will stop caching entries.
Manually configuring this value to something low, e.g 10-20%, based on the current description is likely to lead to unexpected behaviour. The entry cache will be silently ignored/skipped by the server because it is always likely to be using more than 10-20% of the heap.