[Zope] Adjusting ZODB Pool Size

Hi! We have a very busy site that required zserver-threads to be bumped to 10. My understanding is DB connections (pool_size) is hardwired to 7, no "knob" to adjust, and it should match the number of zserver-threads. We do get warnings in the logs, so I should fix it. Create a "custom_zodb.py" and put in your Zope installation directory. Does that mean the zinstance or zeoinstance? The root, or the /etc of the zeoinstance/zinstance? How do I determine the cache_size setting? Right now the "Target number of objects in memory per cache 15000". TIA