I think the reason it's not aligned with the 3d model is that in the first step, you drew the centerline a bit skewed. If you take a look at the vertical line on the "cut out" portion in step 1, it's not parallel to the line you drew. Also, as a side note, the actual pelvis looks like an ellipse when viewed from the top. I think Stan mentioned this in the critique video. The wider this ellipse is, the more the top plane would align with the blue line you drew.

Hey Alfio, I don't want to be controversial here, but, I don't do it this way. The reason is because I hate math, and love to draw. What I do is practice drawing ellipses over and over again until I can do it without a formula, and trust my eye. I am showing you a diagram of how I do it. It is not precise, but to be honest, I don't care because it gets me what I need. Please don't think the way Proko showed it isn't correct because it is, I just am not wired to do it like that. I hope this is helpful in some way :)