Using the 505mm image as an example, my first assumption is that the top of the distance marker is the mid point of the vertical VESA hole distance, so the gap I am looking at would be 505-((V1*0.5)+X2)).
It pains me to say this but, have you asked this on AVF? I’d have thought someone with limited interpersonal skills and a wildly exaggerated sense of self worth will be along to tell you pretty swiftly.