I decided that I'd try an experiment to use the dimensions of the Saint Birgitta's Cap (from the article in Medieval Clothing and Textiles, Volume 4), and use some simple math to calculate a proportionate size to fit my own head.
I measured around my head at the back (at the largest point) to my temples (to where I want the finished front edge to be), divided it in half, and got a measurement of 20cm. Using the measurements from the article of the size of rectangle required for the extant cap of 19cm wide by 22cm tall (as the article provides a range, I rounded the measurements down), I calculated that with a width of 20cm, my cap, in order to have the same proportions would be 23cm (rounded down) tall.
I then quickly stitched it together, included the pleating at the base of each half, and discovered quite rapidly that, well... see for yourself:
Sooo... I'm not sure what to conclude from this, other than that a rectangle of 20cm wide by 23cm tall is obviously too small for my head. I suppose some of the too-smallishness is due to not including a seam allowance, plus some of it may come from not including the gap that's on the original but not in mine. Those aren't really that much in the grand scheme, though.
So, I thought I'd revisit some of the blogs that show recreations to see if I could find any answers. Arachne's Blog was helpful. She discovered that a smaller gap worked better for her cap. I kept the 8cm gap on my mockup, but I think it ended up as part of the forehead to nape measurement (the curve) instead of additional to it. I couldn't actually get the 38cm curve line on the 20cm x 23cm rectangle. That should have been my first clue that the 23cm length wasn't going to work.
Archane's Blog pointed me to this paper. Unfortunately I can't get a great translation out of Google Translate, so I'm not sure what she's trying to show with her diagram.
I think on the next try I'll go with a rectangle of 23cm wide (that's the original 20cm wide plus 1.5cm seam allowance on each side) by 35cm tall (that's the original 24cm plus the seam allowance plus the 8cm for the gap). I have a curving ruler around here somewhere that I can use to get the curve of my head right, since that's another problem I'm having.
On a completely different note, here's the hood I made for Dearg:
It fits him very well and he said that, despite it being a heavy wool with a linen lining, it wasn't too hot to wear all day.
After I find that ruler and do a new mockup, I'll share what I come up with!