How to set a area defined by curve to specific height?

I am try to flatten parts of landscape to specific height. The area is defined by spline. I used Landscape Custombrush landmass, it partiliy worked, but the height of the are is not exlity as same as the spline points. I tryed to adjust the Z Offset but it is not working correctly, the result is affect by Z of landscape and the target height.
Is there a tool to let me controlled the height of an area of landscaple, or is there some setting i forget to set?

Thanks