Proper way of traffic shaping using "burst" with PCQ Queues

Just wondering what others do, and what would be the most efficient/proper way to do this. Understanding “burst threshold” and the “limit / total limit”. We have 50 to 400 Simple Queues on each Mikrotik router, and we use preset “PCQ’s” for limiting the customers down and upload speed. Not sure if that has any effect to the limit and total limit. This is what we want:

EXAMPLE

Burst Rate: 12M
Time: 30 minutes
Rate (Sustained): 4M

After 30 minutes, it drops from 12M down to 4M sustained. Then resets once connection goes idle for a few seconds/minutes.

Now this is how we have it setup:

Burst Rate: 12M
Rate (Sustained): 4M
Burst Threshold: 4M (what is this???)
Burst Time: 00:30:00
Limit: 50 (how does this effect anything)
Total Limit: 2000 (no idea, maybe the number of connections)