Having made a Lytw8 bivy, here's my 2 cents:
I'm taller than Jamie and use a Long bag, so I added (I think) 6" to the pattern length. Good call there; I'm very happy with the length. To keep my costs down by not needing to order another yard of fabric, I lengthened the mesh and head section of nylon by 3" each to achieve that 6". If I were to do this over, I'd add all 6" to the mesh. I get condensation inside when it's all closed up because of bugs. No bugs, I now sleep with the mesh underneath my bag's hood so all my breath dissipates.
I did the chest entry and extended it as one piece down the side by about 30". Definitely a good idea; sometimes I wish I had made it even longer down the side.
I did not add anything to the width, but I wish I had added a few inches, at least in the chest area. I can read and write in my journal inside the bivy with it closed up, but it's kinda tight. A little more circumference would make that easier. I'm not a big shouldered guy, so if you are, add even more.
I didn't bother with any tieouts beyond the one on the mesh to pull it away from my face. I figure I won't be leaving it while I go gallivanting about the hills, so it will always have weight to pin it down in place. My tieout is on the mesh, but another on the nylon would be nice.
I have size 14 feet, so I made the folded corners on the feet 10" (5" sil, 5" breathable). The head I think I made 8" (4" sil, 4" breathable), but maybe it was 6". I could check if you're curious.
Lastly, if you haven't ordered materials yet, I vote that you just make this one as cost effective as possible. I used seconds of everything I could and used NoSeeUm mesh instead of NanoSeeUm mesh. Haven't had any problems with water seepage through the sil 2nds, but I am always careful about site selection. I toy with the thought of ordering the absolute lightest materials from different shops to make my ultimate bivy and to get it under 4 ounces, but I have a hard time justifying that expense. I'm more than happy with my "heavy" 6.5 ounce bivy that cost a quarter of what a 4 ouncer would cost.
I posted mine in the original thread; it isn't anything special in there compared to the others: