The thread for standards for focal spot measurement is in the X-ray tube forum.

A description of the standards is in the forum for Standards, e.g. ASTM standards

An explaination of the newer type of measuring methods with digital detectors is also in the X-ray tube forum.

The slit camera is measuring the focal spot just in one direction, the pinhole camera gives a 2D intensity image.
To get the length and width of the focal spot with a slit camera you would need to turn the camera or the tube by 90 degrees and do a second image.
The revision of the EN 12543 has removed the slit camera method recently.