You are most welcome.
If directions state: space plants 18 inches apart, it means that the plant, when mature, will cover an area roughly 9 inches from the plant center in all directions.
You may want to consider some of the border plants for the area immediately behind the retaining wall. Using either lily turf (Liriope species) or Monkey grass/Mondo grass ( Ophiopogon japonicus) which are both usually evergreen, clump forming, easy care, easily divided plants.
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