Track daily mobile data usage.
A running total tells you where you are but not whether you are in trouble. The pace figure does: it compares your actual daily average with the rate that would exactly consume the allowance over the whole cycle. Anything above 100 percent means the current behaviour will breach the cap, and the size of the excess tells you how much you need to cut back.
Pace against a sustainable rate
Daily average = used / days elapsed; projected = daily average x cycle length; sustainable rate = cap / cycle length; pace = daily average / sustainable rate x 100
Different measurement points and different reset dates. The provider counts at the network including protocol overhead; your device counts application traffic. Discrepancies of 5-10 percent are normal, and a different cycle start date can make them look far worse.
Reasonably, from about a third of the way into the cycle. Very early in a cycle a single large download distorts the average badly, which is why the projection is unstable in the first few days.
The excess tells you the multiple to cut by. At 150 percent you need to reduce daily usage by a third for the rest of the cycle — usually achieved by moving video to wifi and capping streaming quality.