If you do want that angle at 28°, the angles at the joint would become 76/104/76 (instead of 72/108/72), so each angle cut would be 104°/2 = 52°.
In your point of reference, this translates to 180 - 52 = 128°, or 90 + 38°
If you do 40°, you might need to use putty or something to close the 2°...