Tonight, on the Historical Needlework Resources Website, I found examples of three more extant 11th century semicircular cloaks that, because of their richness and provenance, have survived intact.
The "Star Mantle" of Henry II of the Holy Roman Empire is a semicircular cloak with a stretched neck notch like mine.
In contrast, the Mantle of St. Kunigunde, Henry's wife, also has an elongated neck notch, but that neck notch does not have rounded edges; it is a shallow rectangle, with perhaps a slight reverse curve.
Finally, the Coronation Mantle of St. Stephen has no notch at all; it is a simple, if not quite perfect, semicircle.
This confirms my belief that there was no one method for cutting semicircular cloaks in this period; each tailor appears to have done as experience, and (probably) his patron's body prompted. It reassures me that my mantion is of a plausibly period design.