I have a simple sweep 2 rail, its in another post in fact, I dupe edge its bottom edge, project that to Cplane
draw a horizontal line, select the dupe edge one then the horiz and angle says now select second line !!!
I attach that file here…try it, dupe edge the bottom slightly sloping edge in Front view, proj to Cplane, drag it down a bit to see it, draw another horiz line, angle command refuses it.
It should be 1.33 deg
Skin sweep A capture of crease.3dm (120.2 KB)