So I took a few minutes to draw this up in AutoCAD (sorry I’m not up to speed in SketchUp yet). The only dimension I had to assume were the front-to-back rails that intersect the middle of the chair back. I went with 3” because that’s what looked to scale, and gave room for a proper radius. I could’ve sprung for the $20 full-sized plans, but what’s the fun in that? The reason I drew this out is because the back is defined by a 7 degree angle. I thought it would be more accurate to locate the top of the back using Cartesian coordinates vs. a shallow angle. A little bit off on the angle, and you’re way off at the top.
I’ll need to get some masonite. I think MDF might be a little mushy to make router templates out of for so many copies, plus the added depth complicates things. A nice thin hardboard template will allow me to use all of my pattern following bit/plunge with no worries. It’s brilliant to screw the blank to the template at the mortise hole locations (not shown). It looks like I’ll have to do a series of routing along straight edges to make the template perfect, which of course will make all the chairs perfect, right?
-- You can't control the wind, but you can trim your sails