This weekend, I successfully cut a semicircle out of my royal blue fabric that is more-or-less adequately sized for a calf-length cloak. The next step was to cut a notch to accommodate my neck at the center point of the straightedge.
That's when I discovered why semicircle cloaks became obsolete. There is no good way to cut a neck notch in a semicircle of fabric that will make the fabric lie comfortably on the shoulders.
Cutting a notch as deep as the approximate radius of my neck only resulted in odd, pointy corners that stuck out nearly at the level of my ears when the garment was held shut (as it is supposed to be) at breast level. Cutting the notch a bit deeper improved the lie of the fabric at the neck, but caused the hem to plunge in front and rise in back in an odd-looking way.
So I started searching on the Internet for clues about cutting semicircular cloaks. One of the first sites I located summarized the problem well, at least: "If [you] take a semi-circle and divide the straight edge in half, then place the edges side by side each with the center point at the vertex, you will make a cone. But we are not shaped like cones, so do not expect to be completely covered by your semi-circular cloak. There [are] similar geometrical problems with full-circle cloaks. A conic construction will work fine from the shoulders, but between the shoulders we are rectangular."
Now they tell me. :-(
Another site, which focuses on making recreations of the costumes from the Lord of the Rings movies, recommended drawing a full, neck-sized circle about two inches in from the center of the straight edge of the semicircle. The idea is not to cut the circle out, but to draw it as described, cut into the fabric all the way from the edge to the back edge of the circle, cut out the rest of the back edge of the circle, and fold the cut edges of the fabric back. Although this sounds like an interesting solution, I felt I had to reject it as insufficiently historical in design.
A third site, which contained instructions for how to make a Roman paenula (a semicircular cloak with a hood grafted onto it), noted that you need to start with a 20 cm slit at the position where the neck will be. I started cutting back the neck notch on my semicircular piece of fabric, but I was reluctant to cut 20 cm in, because deepening and widening the notch still hadn't solved the hemline problem (though it made the neck fit better).
Then I went back to the Levantia site, where I observed that, as the Lord of the Rings costume site had suggested, the mantion had in fact been folded back and cut at the neckline. So I took my piece, with the neck notch widened from my previous cuttings, pinned back the corners of the neck notch until the fold formed a straight line from collarbone to hem, and tried the result on once more. Voila! The result--though not the most comfortable garment I've worn-- looked more or less like the photograph of the reproduction of the mantion shown on the Levantia page.
So that is how I'm going to proceed. I'm not going to cut off the corners of the neck notch, though, until I've bound most of the cloak. After my debacle with the sky blue wool, I'm a bit nervous about cutting anything else prematurely.