A margin is sensible; a very large one is money spent for nothing. The balanced approach is to estimate from your own recent usage and round up one step. If you are still uneasy, prefer a plan that can be topped up mid-trip over buying the largest size up front.

Generally not. Travel plans have a fixed validity, and whatever is left ends with it. The precise terms sit on each destination's plan page, so read them first if you expect to travel again soon.

Speed depends on which network you are attached to at that moment, how busy it is locally and how well the signal reaches indoors — not on the balance alone. Move, toggle aeroplane mode once, and test again. If it is slow everywhere and at all hours, message the team and tell us which city you are in.

Yes, where the plan permits sharing — that is stated on the plan page. The thing to watch is that laptops download updates and sync work files on their own, which burns through an allowance quickly. Mark the connection as metered before you start.

The signature is that everything still connects but is uniformly slower, in every app and in every location — unlike weak coverage, which improves when you move. Compare what you have used with the terms on the plan page; if the numbers line up, a throttle is the likely explanation.