Why is the flattened surface off at a strange angle? The reference edge is coplaner with the part.
Here is another image of what I am doing.
Can you post your model here? it depends much on the ref. features you used to build your model
The Flatten Surface command is going to flatten the surface starting at that edge's tangency and then pulling the surface up to there. Since your edge is a closed contour the program is picking a location on that edge to come up to but it will still end up making the flatten surface tangent at that chosen point.
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