The room is not the drawing
A frameless shower screen is a set of flat, rigid panels fitted into a space that is almost never flat. Walls are out of square, tiles sit proud of the wall in one corner and flush in another, and the floor has a fall so water runs to the waste. Every one of those differences is a few millimetres, and a few millimetres is the difference between a door that swings clean and a door that binds on the tile.
Measuring after tiling is not caution, it is the only measurement that means anything. Before tiling you are measuring a guess about where the finished face will end up.
What gets measured, and why
The opening across the top, the middle and the bottom — three measurements, because a wall that is out of square is a different width at each height. The height to the finished floor at both ends. The position of the waste and the fall of the floor, because the bottom edge of the screen has to clear both. And the hinge side: which wall the door swings from, and what it will hit when it opens.
Door swing is where cheap screens fail visibly. A door centred for appearance will hit the vanity, or the towel rail, or the person standing at the basin.
Why 10 mm and why hinges rather than a frame
A frameless screen carries its stiffness in the glass rather than in an aluminium frame, which is why the panels are heavier and the fixings do more work. Hinges and brackets have to be rated for the panel weight and fixed into something solid — a stud, not plasterboard.
The glass is toughened, which means it cannot be drilled or cut after tempering. Every hole for every hinge and every bracket is placed from the site measurements before the glass is made. There is no second attempt.
The details that stop water
Water does not travel in straight lines. It runs along the bottom edge, creeps through the gap at a hinge and pools where a seal meets a floor that falls the wrong way. The details that stop it are unglamorous: a seal that follows the actual floor line, a door that closes against a fixed panel rather than a wall, and a screen sized so the water has somewhere to go.
This is why we ask what is happening to the floor before the screen is ordered, and why a screen fitted before the tiler has finished is a screen that gets remade.