Warning Novice at Work.....

Attached are two parts and an assembly in which I am trying to put the two parts together,

Side Outer is a long plank of 4mm thick plywood

Side Spacer is a 100mm Square of 4mm thick plywood

In the assembly I have set mates to mate the faces of the wood that touch and the bottom edge which is all as I wanted it to be.

The problem I am having is that I want the left hand edge of the side spacer inset from the end of the side outer by 4mm (specifically - by 1 x Material Thickness) but I am clearly doing something wrong with the dimensioning as I can get an dumb dimension that says how far it is from the edge but not a smart one that sets it to the distance I want.

Assemblies is new territory for me so I would be grateful for a few clues.

Secondary Question (whilst I am at it)

Once I have the spacer in the right place what is the best way of projecting the holes and slots in the spacer through the outer?

Thanks in anticipation

Paul Martin