A couple of comments on the Kickstarter comments page asked about parallax correction, which is an issue characteristic of all TLR cameras. There are a number of ways to correct for parallax error - moving the entire viewfinder image, Rolleiflex-style, a moving needle to note the top of the frame, a la Mamiya, and finally fixed correction markings in the viewfinder. To minimize complexity, Duo will use the last option.
But the problem still remains of how to calculate the error itself. We could of course measure it empirically by shifting the camera up and down by the lens separation of 75mm, or by comparing the VF image to the one projected onto ground glass in a roll film back. Either would work, but why not use some math to characterize the parallax error throughout the entire focusing range?