trumpet86, I am not quite understand what do you try to do, but this is how the formula works. For example if the outside radius of the cylinder equal to inside radius + the thickness of the wall, then what you should do is:
create a parameter for the wall thickness
create a parameter for the inside radius
create a parameter for the outside radius
After you create those parameters listed above, double click on the outside radius parameter. When the "edit parameter" pop up, click right mouse button on the "parameter's value". Next click "edit formula" then you just need to select parameter of the inside radius (+) wall thickness parameter. Try this out you will see the outside radius value changes when you change the inside radius or the wall thickness values.