If you build the two slopes using a fixed value and a fixed width flat bottom, the distance between the two roads and difference in elevation will 100% control the depth and location (x,y) of the ditch. To control a ditch depth, you need some type of variable width or slope segment on one side or the other (or possibly, both).
What you can do, is after running the corridor with this fixed ditch, is to project one of the flat bottom 3D features to one of the road profiles. This will show you what the ditch does based upon those constraints, compared to your desired profile. Then you can use that information to develop components with controls that allow you to vary the ditch depth.
You might want to make a null point that the ditch bottom elevation is tied to and add the two variable segments mentioned above and by using the move point tool, move the null point around to see how it modifies the ditch. You will usually learn where you must free any additional constraints yo get the desired results.
What you can do, is after running the corridor with this fixed ditch, is to project one of the flat bottom 3D features to one of the road profiles. This will show you what the ditch does based upon those constraints, compared to your desired profile. Then you can use that information to develop components with controls that allow you to vary the ditch depth.
You might want to make a null point that the ditch bottom elevation is tied to and add the two variable segments mentioned above and by using the move point tool, move the null point around to see how it modifies the ditch. You will usually learn where you must free any additional constraints yo get the desired results.