Hypothetical use of a measuring tool based on a staff.
The vertical and horizontal bars are marked in the same way as a conventional staff (5 1/2 tesserae multiplied by 1.5). The diagonal bar falls along the line of the centre tesserae on the 45 degree line, this is the width of 5 tesserae.
This allows the marking to be faster and more accurate on wider borders such as the 6 strand guilloche.
It's possible that something like this could have easily made up on site and the measurements taken from the mosaicists staff. I would think that it would just be made to be the width of the border so it would be smaller than this version.