From WHM's Service Status screen, how is "Memory Used" calculated?
The value is calculated using the following formula:
"Memory Used" = MemTotal - MemFree - Cached - Buffers
And the values in the calculation are pulled from the /proc/meminfo file. You can view them using the following command:
egrep '^MemTotal|^MemFree|^Cached|^Buffers' /proc/meminfo