How to choose a correct value for open_file_cache in nginx configuration?

I have a website with load of about 60 000 requests per minute served by nginx. Recently, I have enabled the open file cache and saw a dramatic improvement in performance. But in the evenenings, when loads is maximum, response times are still quite large and nginx uses a lot of IO.

Here are my current settings:

open_file_cache max=10000 inactive=30s;
open_file_cache_valid    60s;
open_file_cache_min_uses 2;
open_file_cache_errors   on;

And I still have some memory left:

free -m
             total       used       free     shared    buffers     cached
Mem:          7910       6917        992          0        439        885
-/+ buffers/cache:       5592       2317
Swap:         8099          0       8099

Question: now do I choose correct value for open_file_cache max=10000 and other parameters? Is 10000 enough, is it too small or big? Is there a way to monitor open file cache use?