I will post steps later today, but the key is -
manually calculating a beginning flat is fraught with errors and too much like work (I avoid work).
You know what the finished dimensions are - so model to the finished dimensions (in this case a simple Contoured Flange) and then add the cuts.
The cuts can be done in one of two ways - use the Unfold/Refold as suggested earlier (Note that Unfold/Refold is not the same as Fold, just as Flat Pattern is not the same as Unfold/Refold or Fold - all of these are different tools).
The second technique is Project Flat Pattern - which I will demonstrate in a few minutes. Back in a bit....
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