dotNet brush Slider

I try to simulate a slider that will be able to control the value of the Spinner, and vice versa. Everything works nicely if the minimum value of the Spinner = 0, but if the value is less than zero then there is a problem. Here's my code. Is there a simple solution to solve this problem (for example, the minimum value to be -400 and the maximum 600).
Sorry,for double post. Here's my test solutions.

dotnet_brushslider.ms6.3 KB