The guy that does my powder coating has a chart that shows "gloss" in 10 percent steps from flat to gloss. Semi-gloss ranges from 20 to 80 percent with 20 to 40 percent being the preferred and recomended choice. My paint guy claims he can do the same so ask your paint store if they can mix a 20 to 40 percent gloss. Don't be too pickey, there is a lot of "lee-way" in semi-gloss anyway.
Jim