hey dazco
I've used the finishline chaincleaner tool, and the park tool cyclone, and I think the finish line one did a better job, but it broke after a while and i think the park tool one is more sturdy. However you're right it does take a lot of chainbrite to fill, and you don't want to be chucking it all away. I would recommend, after cleaning the first time let the cleaner rest for say half an hour, in this time, the magnet and indeed gravity will sink everything to the bottom. At this point you can pour 95% of the clean chainbrite out into a container, then get rid of the mucky 5% at the bottom, rinse out and refill it with the 95% + top up and you're ready to clean the chain again.
In the long term having a jam jar or something with a lid that won't get dissolved by the cleaner is a good solution otherwise it will just evaporate if left in the cleaner.
Hope this helps,
Simon