I placed a pin to the center of one side and tied a cord to it. I made a knot at the right length and started marking half the circle with a charcoal. It's annoying that almost every string stretches. Maybe I might try the one masons use:)
As a concept of a future neckline I made the same in smaller version.
After some trying on I got about this pattern for the darts. They are necessary as the cloak's not made of a full circle and it would open in the front.
I had to cut out the edge a bit - I found out it's perfect when you take some pattern of a neckline, apply the back part to the part between the darts and the front part to the right and left - 90° turned.