I've used xcache and APC. This question isn't about which is better.
I'm wondering if there's an optimum ratio of free to used memory. For instance, let's say I set the total memory available to 160MB, but XCache ends up using 155MB with 5MB free. Is this good, bad or neither? Should I make available enough memory so there will always be a healthy buffer of free memory? And if so, is there a free to used ratio I should hit, or an absolute number of free MB I should always try to make sure is available?