To know when to add more SSD the Automated Workload Analyzer (AWA) feature can also be helpful. In the same TR mentioned earlier, section 6, is discussion of it.
If you want to see this graphically, and lots more, then you could also go for the advanced performance monitoring solution of Harvest + Graphite + Grafana. I wrote a blog here that has a video of what you get and the steps you need to take to make it happen. It could be overkill just for this cache monitoring, but if you want more details across the board it could be a good fit.
Here is an example of the flash pool stats it captures (pardon the boring graph and crazy lines when there is an IO but the SE lab is pretty idle):
The AWA panel is blank because awa hasn't been started on an aggregate. If you start it (see the TR for details how) you get something like this (from a different lab system that is busy but has no flash pool today):
So AWA is telling me if I had ~400 GB of SSD I would serve 81% of read IOPs and 42% of write IOPs from it. We can also see that over time it is relatively steady, so my active data size is stable and not influenced by some large batch jobs or similar activity.
Hope this helps!
Cheers, Chris Madden
Storage Architect, NetApp EMEA (and author of Harvest)