Set alerts for data usage thresholds.
Alerts are only useful if they arrive with time to act. Spacing them evenly across the allowance — dividing the cap into one more interval than the number of alerts — puts the last warning below the cap rather than at it, leaving room to change behaviour. Comparing the projected alert day with your current daily average shows immediately whether the alerts will fire early enough to matter.
Even alert spacing
Step GB = cap / (alerts + 1); first alert = step; projected day = first alert / daily average; safe daily = cap / cycle days
So the highest alert lands short of the cap. Three alerts across a 100 GB plan fire at 25, 50 and 75 GB, leaving a quarter of the allowance as reaction time. Dividing by three would put the final alert at the cap itself, which is too late.
Most mobile operating systems have a built-in data warning and limit, and most router firmware and mesh apps support usage alerts. Provider apps often only offer fixed percentages, in which case round these figures to the nearest available option.
Then your current pace will not reach the first threshold at all, and the alerts are effectively inactive. That is a sign the allowance comfortably exceeds your usage.