It's a little involved to get a live number of the amount the network believes to be staking. Basically, it would be derived from the staking difficulty target, similar to Chain Power, described in the proof of power document a few messages above, over the last 10 or so staking blocks. It can be derived, and if you want to write a python script or something, we could help you. Otherwise, the proof of power doc is something that could give you a starting point. We will calculate and expose that number in an upcoming release, as a lot of people would like to have it easily available.