Line A is where the CG of the laptop will be at. If the CG of the laptop was on line B, there would be very little bending moment on the back pillars. The bending moment on the back pillars will be weight of the laptop multiplied by distance between A and B. However, for best overall stability of the structure, the CG of the laptop should be ideally above line C.
We were most worried about the bending moment(BM) on that back pillars. This BM will affect the pillar joints. That is why if you notice we bought line B closer to line A by attached the pillar closer to the center of the top deck.
Typically most other laptops are larger and heaver that macbook air. Larger laptop means line A will be closer to B. So for most laptops the bending moment on the back pillar will be more or less the same. If you have a really big laptop, then line A could end up on the other side of line B, in which case your entire stand will be unstable.
Tested with Macbook air and Asus k53