As I said there is no solution for this as there is no API for it and it depends on your network speed between your client and the server as well -> ping.
I can't think of any way to determine via addons if you are able to start a light attack.
Hi everyone,
How to know if you can light attack ?
There was a add-on which monitors the global cooldown. Maybe this delivers a helpful opener.
If not - as Bertram said, you have to calculate this time point.