(For these screenshots, the pink wall and red floors represent the FOF's control sector/linedef, and the black floors and blue walls represent the sector used to slope the control sector.)
The tricky part about FOF slopes that you have to keep in mind is that the slope stays completely coplanar to itself no matter where it is in the level. Here's a demonstration:
Notice how the FOF's top stays perfectly lined up with the control sector's ceiling. This means that the position of your control sector relative to the in-map FOF is very important and must be accounted for. Watch what happens when I move the control sector, while applying no other changes to the map (including the shape of the control sectors):
The top of the FOF got lower, because the way the control sector was moved made the slope lower in global space, which means the same X and Y position has a lower Z position than before. When you're trying to build FOF slopes with the linedef-based specials, the easiest way to line them up is to place the control sector somewhere perpendicular to the target sector, relative to the direction of the slope's plane:
Unfortunately, this isn't always feasible due to how levels are structured. (Or sometimes you just want to slope both planes in different directions!) What I do in these cases is use Visual (3D) Mode, fly out to the control sector and select the plane of the extra sector used for sloping. (So the sector that isn't the FOF's main control sector.) Then I can go back to the target area's FOF and scroll the mouse wheel for plane heights:
with the FOF's plane selected to move the entire plane up and down
with the FOF's plane deselected to adjust the slope's angle (small changes tend to get magnified when doing this, so holding shift for precision helps)
