Free memory, include "cached"?

Hello,

I have to report the amount of FreeMem on my embedded system. The rootfs is in RAM. I use /proc/meminfo.
I started by doing : freemem = MemFree + Buffers + Cached But I realized (tell me if I'm wrong) that my rootfs filesystems is counted in "Cached".
Part of the memory in "Cached" can be allocated, so I should add it as Free Memory, but part of it is my rootfs (i.e. really used). How should I compute freemem ?

Note that with linux 2.4 (initrd and not initramfs) I did use the ioctl BLKGETSIZE in order to subtract the amount of memory used by the rootfs. But now with initramfs it's dynamic.

Thanks,
Greg