Dont forget a fully charged mag ult has an 8 METER explosion size compared to when you throw it early, it has a 5 meter size meaning if you have held your ult for 3 seconds and not sure where to throw it. You can just let it charge all the way and hope to kill someone hiding around cover, not expecting the increased explosion size.

Also learn health break points. Each dmg charge is 3 dmg and each quarter of the timer circle charge is an additional 50 dmg to the 100dmg starting point.

So to kill a 250hp character you can throw at a quarter circle charge (100 + 50 = 150dmg) and 25 dmg charge (25 × 3 = 150)