In a serving glass, combine concentrated coffee with lemon juice, sugar, and salt, stirring to combine and fully dissolve sugar (if not using simple syrup). Taste, then adjust flavor as desired with additional lemon juice and/or sugar until the flavor is strong but properly balanced between sweet, tart, and coffee. Add ice, dilute to taste with cold water, and garnish with lemon rounds. If desired, spike with rum to taste. Serve.