I think the problem is that the program rotates cross-sections' drawing planes around the cross-section centers. As you can see in the picture the plane is perfectly diagonal in the corner but its grid is rotated. The grid should be perpendicular to the imaginary plane of the lines around the corner or one should be able to rotate it. Then one could calculate the change in the form by hand. For example in a corner of 90 degrees the cross-section is in a 45 degrees' angle to the paths' sections and its width or height(whichever changes) would be divided by the sine of 45 degrees (The value is bigger than the original). I managed fix some of the corners in this pipe I created with a cross-section in every corner by doing the corners the way I explained( with dividing by the sine of 45 degrees). In the other corners the grid was rotated and changing the width and height values would have only deformed the pipe in wrong directions. I also tried to set the values first and then rotating the shape but it is hard to get it right that way too. Anyway one shouldn' have to do this kind of shapes by hand. Hope this helps you even a bit.