Von Little teapot:

nncache saves need to query NN for transpositions (increasing nps slightly).
Every nncache entry takes 350 bytes of RAM.
Every node in the tree takes 250 bytes of RAM.
You have to decide yourself whether you want to allow faster search (by increasing nncache) or longer search (by saving RAM by reducing nncache).

You can use the formula:
```
max_NN_cache_size = (total_RAM_in_GB)*2500000 - (Longest_move_time_in_minutes)*(nps)*43
```

Lothar