Here's the scarfing jig I ended up with for dimensional lumber (up to about 3"). It is 12:1.
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I can't take credit for it, after messing with ideas yesterday I knew what I wanted to do with my sled and thought I wanted hold-downs, but hadn't figured out my clamping plan. Came across some plans last night on Fine Woodworking that fit the bill perfectly. The fence is screwed to the sled at the desired angle, with bolts running up from its bottom (in my case, the bolts are buried between the fence and the sled). A piece of scrap the same height as the piece being scarfed is set (loosely) along the back of the fence. Hold-down blocks sit on top of that scrap and the piece being scarfed. That pressure holds the piece being cut square to the sled and fence. The only really limitation is height... this works up to the max height if your table saw blade, less the sled thickness.