Firstly thanks for taking time and completing this exercise!

About your Rough Perspective: Its getting better! You are on the right track. Just remember that your front and back faces of boxes need to be rectangular(composed of horizontals that are parallel to the horizon line, and verticals that are perpendicular to it) and everything should come in place with warmups.

About your question: You already using it, kinda :p. Look, in Rough Perspective exercise your choices ends after you place first of the back corner. Then, comes two rules: first that your box is converging towards your vanishing point, meaning edges that connect front and back faces will go towards vanishing point and second that front and back faces of the boxes need to be rectangular. With those two rules in mind you start to apply ghosting method to find where your remaining corners will go based on the information present on the page so there is no arbitrary guesswork (https://imgur.com/a/eLREsaW). Only after you make sure that two of this rules you think are applied you start to connect corners and make marks. In Organic Perspective exercise, on the other hand, we don't have that much information - we only have three edges of y and besides that we have no only one but three vanishing points. So here comes the negotiating corner when we need to find a compromise of the two pairs of edges in one face of the box to make sure that they converge to corresponding vanishing points. And besides that we have plenty room for our choices: make box with dramatic or shallow convergence. So in Rough Perspective exercise you use a simpler version of ghosting method then in Organic Perspective. Don't worry you will have a lot of time to experiment with those methods in 250 box challenge.

As about the 250 Box Challenge: