Like you I've never been able to get all my triangles like this to fit the same. Using a paper piecing method would help a lot (fold the sky section back & sew next to the fold). Or else use 5 1/4" squares on each corner (snowballing), draw a diagonal line & sew inside the line, then 1/2" from that line, and cut the corner off. You will have a decent size HST for use elsewhere.
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stitch on the dotted line, cut along the blue line.