I finished sewing all the panels together, and trimmed off some of the red ribbon. It was just too difficult to get an idea of the overall effect with too much red.
Then I trimmed the panels, leaving around 1/2 an inch extra each side. I turned the panel to the reverse, then marked the line I was aiming for in pen. I folded and pinned the edge, which was a real pain since there was so much seam in the tightly packed panels.
I'm really happy with the shape, it's actually worked better than I had hoped. The photography could be better, but that's pretty normal for me now.
And I'm rethinking how many poppies I need now, probably could do with twice as many.