Any builtin threshold of used cache size?

[askformydream]

I am using fastcache in a golang-based web service which captures images from multiple cameras with settings:

• camera count: 8
• FPS: 7
• resolution: 2048*2048
  When I set the max cache size to 384MB, 512MB and 1024MB, the percentage of used cache size is always about 57%.
  Is there a builtin threshold for this cache usage limitation or are there some reason for cause this percentage?

Thank you

[askformydream]

Additional test:
When my program store 1.5 million image in the cache with size 1024MB, the maximum usage percent is 61%, and there are 15 image evicted.
So my question, since there is much free cache space, why the images are evicted?
Thank you