Hello. I'm currently studying Loomis' Successful Drawing. I'm currently going through the perspective section.Drawing to scale to be exact. I've attached the page on how to do and indicated the lines I'm having a problem understanding. The line is (O,C). If you read the page it will give you a better understanding of what that diagonal does. In addition to that diagonal, I don't understand how to repeat the process and achieve the additional measurements. I also included my work. The diagonal is on the drawing, which I achieved by going through the vertex of the "optional depth of first foot" line. Which I don't think is correct, because I can't replicate the distance of the depth of the optional first foot.
Hope I explained my problem clearly.