I used extrude to cut a shape out of a series of ‘ribs’ atop a flat base; the new edges of the ribs are at wacky angles, due to where they intersected with the shape i used to cut through them. I want to make those edges perpendicular to the sides of each rib (i.e., 90 degrees)-- but when I tried to select the angled face and use the "rotate around axis’ to move that face to 90 degrees, it moved it to some other angle instead-- and when I typed in “90”-- it pushed the angle in the other direction. IOW, “90 degrees” wasn’t “90 degrees” for some reason.
A picture is worth a thousand words!
Screen grab what you are doing, or a similar example