You must go in deep of mangle and queue tree to do it.
Then you can: mangle your traffic using "Connection-Bytes" property, then mark packets in different mode if they are <300MB or >=300MB.
Queue this packets in different speed queues.
To reset the counters each day, you can script at certain time (i.e. 3.00AM) to disable ad re-enable connection tracking (but I am not shure of this).