Cisco ASA: Memory Usage

I read recently that if the ASA is using 80% or more utilization, then you need to upgrade the memory in the box.  I agree, although I might have said 75%.  So how do you see how much memory usage is taking place?

assa# sh memory
Free memory:       334483928 bytes (63%)
Used memory:       195206568 bytes (37%)
-------------     ----------------
Total memory:      529690496 bytes (100%)
asa#