Set a monthly data usage budget.
A usable data budget is not the full allowance divided by the days in the cycle, because that leaves nothing for the unexpected large download at the end of the month. Holding back a deliberate buffer first, then dividing the remainder, produces a daily figure you can actually run at without anxiety. The per-device share is included because household disputes about data are usually about fairness rather than the total.
Budget after buffer
Usable GB = allowance - buffer; daily budget = usable / cycle days; weekly = daily x 7; per device = usable / devices
Around 10 percent of the allowance covers ordinary variation. Make it larger if a single foreseeable event — a game download, an OS upgrade across several devices, a visitor — could consume a big share in one day.
As a reference point rather than a rule. A smart speaker uses almost nothing while a 4K television can consume most of a plan alone, so the fair-share figure is most useful for spotting which devices are far above it.
If your provider offers rollover, yes — then an unspent buffer becomes next cycle's headroom rather than being wasted. Without rollover, an unused buffer is simply allowance you paid for and did not need.