I have a box and I need to cut something out from the side. The indent must be equaly spaced between the perpendicular sides.

Like so:

|               |
|               |
|               |
|     _____     |
|____|     |____|
I have created a reference plane in the between both outer planes. I have created a block the size of the indent but I cannot seem to figure out how to align it perpendicularly centered to the reference plane. I do not want to eyeball it obviously, it must be perfectly centered. Mating doesn't seem to work. I have tried creating another plane between the top and bottom and tried to sketch a centered rectangle but I cannot snap to the intersection of the planes.

What is the trick to do this.

Please note that the geometry is more complex than what I have shown here so simply sketching this shape and extruding it is not an option. The sketch above is simply for reference purposes and illustrates my issue.