You have a little bit of hip roof framing going on here and that's a bit complicated.
What you need is the angles for the saw cuts which requires much dimensional information, i.e., runs and rises of the material.
There's a non math option, I'd make a set of, (1/4 or 1/2) scaled test pieces 1st, to minimize waste. Take some scrap plywood, and draw 1/2 your seat backer with the 30° on 1 side and do the same for one of the side pieces. Rip the 30° along the bottom, (plate cut) of both pieces take the cut offs from the side cuts and use them to prop up the backer and piece at 30°. The 30° side angles will be cut back later, they're always less than the 30.
Use a framing square, plywood or some other thing to create a 90° right angle, (pseudo seat) so you can slide the 2 pieces together at 90°. When the tops, of the 2 pieces touch, (ridge cut) mark the bottoms of both pieces where they stop along the 90° corner.
1. Measure this distance off the 90° corner to the marks made, e.g, (X) = 3/4".
2. At a 90° angle measure (X) off from the point where the backer and side piece touched, (ridge cut) and mark both pieces.
3. Strike a line from the bottom, (plate cut) to and through (X), (new ridge cut)
4. Use speed square or whatever you have to discover the new angle
5. Cut new angle into both pieces and test on 90° corner, should line up perfectly for butt joint.
I'd go with a butt joint for screwage, if you're going for a miter cut it should be at or very close to 45
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