Thanks Brion,
3/4 extension appears to be just right. I started by building the inners since those would give me the least amount of scrap material if I messed up. and the technique I used managed to get the two shrouds within 1/16" of each other, and very close to my target length. I think I ended up 3/8" short of my intended length even after pre-tensioning the splices on the bench, but upon installation and tuning, I am very happy with where the turnbuckles are sitting. They have quite a bit of take up left.
Brion, your oak handle marlingspike works brilliantly for creating the Brummels. The taper allows the line to be opened up just enough to pass the bitter end through, or wide enough to pass the eye through.
With the inners installed and tuned, The turnbuckles are at a comfortable 2/3 extension.
I hope to be able to take the mast down this weekend to build and replace the rest of the stays.