How to Calculate Pro SItes storage limits when using a CDN for wp-content?

Hi Folks

I think this is going to require custom coding but want to check first, We use a CDN to store all the uploads on the multisites wp_content, this is set up correctly and working fine. As far as I can tell Pro Sites only looks at the local wp content folder even when the multisite is setup with wp_content on a CDN. Obviously we need to be able to track the usage to warn users they need to upgrade to pro on reaching the free limit and also when a Pro User reaches their limits (defined in the Pro SItes plugin).