Limiting slope on through lane while curb profile is variable

I can't help but feel there is a simple solution for this but here I am, in need of the great wisdom of the community here. 

I'm trying to get my road shoulder slope to vary while the slope from my centerline to the edge of travel lane fixed at 2%, while my gutter inslope from edge of pavement to the gutter flowline is fixed at 6%. A vertical point control will be placed on the flowline so the curb will move vertically depending on the curb profile. 

The road shoulder should never be flatter than the slope of the through lane. If the road shoulder needs to be flatter than 2%, the through lane will match the road shoulder slope. 

The two not-really-solutions I could think of are:

1) Setting a zero vector-offset between the centerline and the edge of pavement but my through lane slope will be matching the road shoulder slope when it is steeper than 2%. 

2) Setting a slope constraint of 2% from the centerline for the through lane but the shoulder could be flatter than 2% and cause a dip. 

How do I get the shoulder slope to vary so it is 2% or steeper when the curb profile is low enough, and also able keep the through lane slope to match the shoulder slope whenever it gets flatter than 2%?

Thanks for any help in advance!