How to calculate available memory when zram is used?

[yong w]

How to calculate available memory when zram is used?
Available+Swap free memeory is obviously bigger than I actually can
use, because zram can compress memory by compression algorithm and
zram compressed data will occupy memeory too.
Is there any way to calculate available memory accurately?

I think of a way is to count the compression rate of zram in the
kernel. The available memory  is calculated as follows:
available + swapfree - swapfree * compress ratio
Is it reasonable?