I don't think you are taking on "too much" by placing the boxes in here. There are a lot of great reasons to find the box. But to give a little bit of my personal experience with boxes, they are only helpful when placed in the correct position. So for example, the top corners of the box representing the hips should perfectly align with the PSIS and ASIS. this can be challenging to do from a photograph, but not impossible, We can often see at least some indication of the PSIS points or ASIS points, but usually not both so we have to go on the tip and tilt of the trochanter to accurately assess the side we can not see. The bottom of the box for the hips will align with the pubis and the triangle of the sacrum can point us in the right direction. This can be challenging but a really great exercise in seeing beneath the skin and accurately finding those landmarks so that you can place those boxes in the right place. For the ribcage, you can find the 10th rib with pokes out on the left of the model on the second photograph in your reference, and you can use the manubrium (if seeing the front of the model) to judge the tilt. I don't know of an accurate way to judge the top of the box on a model from the back. potentially at the 7th cervical vertebrae? I find these boxes super difficult to place in accurately, but what they can do is help you understand what planes of the body we are seeing.