I want to create a queue setup. I am marking download type traffice using mangle in pre-routing. I want to ensure that this download traffic get lower priority than all other traffic. That way when I do other things the download traffic gets throttled back.

There are examples that show this but I was wondering if there was something for when bandwidth varies considerably. My internet bandwidth is not stable and varies quite a bit. I'm not sure how to just tell the router to drop download packets first.

you have to play with limit-at and max-limit: when every queue reached its limit-at, the remaining bandwith is distributed following priority and max-limit
max-limit can be higher than real bandwith, as routeros flushes queues proportionally: I suggest putting it at the teoric bandwith you should have from provider, that in real life I never reach (in Italy connections are ########).

Thanks.