What about those slanted writing desks and writing slopes. They have a 45° angle, so less than the one Johnston uses in the picture. Think either an old school desk, or that portable slope Mr. Darcy uses in Pride and Prejudice.
Edward Johnston wrote with edged nibs not pointed ones, therefore Salman (reply #4) has described the situation perfectly; Sloping surfaces for the best control when writing with an edged nib, and a flat surface for writing scripts with a pointed nib.
The slope surface used by Mr Darcy was probably chosen for visual effect.
A few years ago I wrote a few pages in early script for use on a period BBC TV production. I wrote these on a flat surface with a broad-edged nib.
When they eventually appeared on the TV production, they were shown in a scriptorium, apparently being written by an actor on a sloping desk.
On TV accuracy is less important than how it looks!