I am not sure I can give you a definitive answer but my understanding, confirmed in this thread by the much more knowledgeable jojothehumanmonkey, is that it is a soft-limit. It limits the files such that there is less than that size but if a single file is larger than the cache (and/or multiple files are open?) it can grow.
Honestly, 800Gb just sounds like a bad idea via WebDAV. You will need to have a rock solid connection including no weak links and be certain you have set proper timeouts as you can quickly break that before it is uploaded.
You may be able to put rclone chunker in front of it though I am not sure if that will solve the problem. I don't recall the retry logic.