There is no standard. Some brands have drawn a scale next to the tracks, but not all. Some are counting centimeters from the end of the tracks, or + - from the center of the box. Pretty much all different mast plates have different length BTW, so adding the ruler markings that would always make sense even for different foils is not an easy task. And comparing the mast position with others only makes sense if you also list all the foil components that you are using, because changing a fuselage, a stabiliser, shims, front wing, etc. - each one of these changes can possibly change the optimal mast position. IMHO it would make the most sense to mark the center of the mast plate, and the center of the foil tracks.