A common question I used to get asked was “why don’t you put measurements on the tracking rule?” My answer was that they would always be wrong. It actually reveals something deeper about perspective drawing: it is impossible to draw to scale in perspective. This is the whole thing about perspective: the scale is different everywhere on the page. The further away an object is the smaller it is drawn on the page. The scaled used is different to something closer. In fact, the scale changes on different parts of the object itself. This is the very thing that makes perspective work, but it means that it is very difficult to draw something accurately. What you really want in perspective is to draw so the object is the correct proportions. So how do you get the correct proportions without a scale? The VP3 Calculator is the answer. If you know the overall dimensions of the object you want to draw, then the calculator will give you the lengths of lines you need to draw on the page.
It’s a really helpful innovation that adds so much value to the VP3 system.