re: Tea box

No worries. she’ll love it anyway!!!
The way I was taught to do miters, was take a long piece of wood, cut it at 45 or as close as your miter gauge will let you, then take the two pieces and match them together so you have a 90 degree angle. then take the square and check for square. if there are gaps around the square, then either slightly change the angle one way or the other, then recut the angles again, until the angle is perfectly square.
Ive tried using angle boxes, accurate miter gauges, nothing seems to work as well as just tweaking the angle of the saw blade. sorry its not a great description, but I’m sure you can search the net to find a youtube video that can show you.