The screen width is not fixed on QDOS computers, QL Emulators and future hardware expansions (graphic cards) offer different screen modes with different resolutions. The function SCRINC returns the screen width relating to the screen upon which the given channel (default #0) is located. The width is returned as the number of bytes needed to store a line of pixels.
The standard QL mode 4 and mode 8 always return 128 = 512/4.
However, it is not always true that the number of bytes required to store a line of pixels is equal to the number of pixels DIV 4 and you should therefore use this function or similar.
See the second listing at the second example for SCRBASE.