Is the seat line a set point on the coat draft?

I've just finished my second draft now using my model's measurements (Chest 36, Waist 30, Height 72").

Point 4 on the draft is 8" down from pt3 or 5" up from point 2. And then line 4 becomes the seat line.

Is this decided from the waist measurement I used for the draft or should I have used my own model's seat measurement at some point? It just dawned on me that I took this measurement but didn't specifically use it.

I'm presuming point 4 has been plotted in relation to the rest of the measurements I used and therefore should be at the right point on my model but I just wanted to check I haven't missed something before continuing.

Thanks!