graphs and memory

Hi all.

This is old question, but I didn't see clear answer yet.
I try to draw memory usage graph in rrd tools. My server without swap (4Gb is enough, IMHO).

First graph of physical memory. In kernel 2.6.14 Looked like:
Cached+MemFree+Mapped+Slab=TotalMemory
But is not exactly equal. For instance now Cached+MemFree+Mapped+Slab-TotalMemory=22680 This means I use 22Mb more memory, then I have total.
Is this correct? What do I need add or subtract to get total memory?

Second graph is cache memory. For early kernels looked like:
Active+Inactive=Cached
But this is not true now, 2.6.14 for instance. Active+Inactive-Cached=157044 (153Mb)
What do I need add or substract to get full picture?

May be some kernel guru know answers?

Olleg Samoylov