Checking data usage accurately

Trust the provider's, since that is what the allowance is deducted from. The device counter is better for trends and for finding heavy apps.

The device counter works offline; the provider's figure needs a connection to refresh. Use the device counter to decide while offline.

Usually not. The two sides count differently and update on different rhythms: the handset counts what the operating system sees, while the provider assembles figures from network systems that can lag. Watch the trend rather than the second-by-second value. If the gap grows too large to be explained by that day's activity, gather the evidence and get in touch.

No. The handset does not keep a history of what you cleared. All that remains is whatever running total the provider can show you. That is why it is worth screenshotting the screen before every reset, so you still hold a reference point of your own.

Because the phone works in the background whenever it sees a connection: syncing mail, updating apps, backing up photos. If the accommodation Wi-Fi drops in the night, that work moves to mobile data by itself. Restrict app updates and photo backup to Wi-Fi, and leave messaging and travel apps free to run.

All of it counts against the plan on the phone doing the sharing, whatever the other device does, and the per-app screen will not break down what the laptop spent it on. Before sharing, turn off automatic updates and file sync on the connecting device.

Once a day is enough for most trips, at the same time each day so the readings are comparable, before bed for example. Checking constantly mostly means reacting to figures that have not settled. On a tight allowance, or when several devices share one hotspot, add a midday check so there is still time to adjust.

First stop the heavy things that can wait: photo backup and app updates. Then lower video resolution and disable autoplay in feeds. Only then look at what you genuinely still need until the end of the trip. If it is still short, consider an additional plan sized by the days and the activity left, rather than by alarm.