For large pieces like you're working with, I have two right triangles my father made of oak years ago. The two adjacent legs are about 10" each. I lay the pin board on my bench and clamp the triangles to it. Then I clamp the tail board to the triangles. A little loosening of the clamps and wiggling of the boards lines them up for final clamping. Once the clamps are tightened, the the boards are securely in position and I can mark the pins. The triangles come in handy for other tasks as well.
I have also clamped the pin board to the front edge of my bench, flush with the bench top. Then line up the tail board, clamp it to the bench top with holdfasts and mark away. This method is simpler than using triangles, but I've gotten used to the triangles and use them most of the time.
Hank
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halfathumb,
I'm a tails first guy, but the photo shows what I do; you can modify for pins first. It will also work for half blind dovetails.
A straight edge board is clamped to the scribe line on the tail board. This board acts as a stop to aid in scribing the lines. In the photo only one clamp was necessary, but sometimes if the panel is bowed two will be required to flatten the board. With the boards up on edge, clamp the pin board to the tail board, and scribe the tails. Again, sometimes if the boards are bowed a couple of clamps maybe necessary to pull things together.
Rob Millard
www.americanfederalperiod.com