Add global equation in limit distance mate?

Hi,

I would like to add a global equation in a limit distance mate as when I change the assembly dimensions the limit distance mate needs to change as well.

When I double click in the limit distance mate in the feature tree and then double click on the dimensioning to ad the global equation value ="GUIDE RAIL OFFSET" I get an error saying. "Please enter a number greater than or equal to -100000 and less than or equal to 100000

SolidworksAssemblies