Uniform Offset of Surface to Split Solid Body

[chawl017]

In this model I am attempting to create two seperate split bodies cut using two individual surfaces, one offset from the other. I want there to be a consistent and uniform gab between the two split bodies. For some reason, while using the surfaces to cut the body, the side and bottom profiles of the cut stay relatively uniform but rotating to the top of the body shows there is a distincly much larger gab between the two surfaces. 

I have tried offsetting the initial surface in both one direction and two directions, thickening the surface and cutting the body using the combine tool, using the push and pull function on the seperated bodies and all have given the same result. 

Making the end of the cutting face more perpendicular to the face of the body does reduce the gap, but there is still variation when compared to the side and bottom profiles of the cut. 

Any ideas would be much appreciated, thank you in advance.
 
(The images below show a simplified test model)

[etfrench]

You're not measuring it normal to the curves.  If the model surface was normal to the curves, then the distance would be the same throughout.

[aliobidi]

Hi , 

attach your model file