Hi Dan,
Yeah, I always fool around with these and try to figure out how it is being done, but never get there. Here's a file of the process I used that also includes a surface from Rhino that unrolled the same solid. I failed with mine to match it. Though when researching this type of thing, I always find it being asked, and many answers are "it is not precise". Even Rhino's output is off a bit from the actual edge.

MoI's edge = 18.7470243394886

Rhino's unrolled edge = 18.7500218509622

I was stuck on developing the "Hieght" properley to have the curve edge be the same as the solids. AN interesting thing I found was that a Conic Curve is created from the 3 points. I was trying to fool around with how to staighten that out to flat. Usually I have found though, that when Michael tells me I beating my head on a tree.... I have a headache at the end of the day! AND I DONT KNOW WHY :O