get_progress: cleanup case statement

app/models/target.rb

@@ -28,16 +28,17 @@ class Target < ActiveRecord::Base
   # ====Returns
   # * +String+ -> progress in percent
   def get_progress
-    case unit
+    numerator = case unit
     when Target.units[:submissions]
-      numerator = conference.events.where('created_at < ?', due_date).count
+      conference.events.where('created_at < ?', due_date).count
     when Target.units[:registrations]
-      numerator = conference.registrations.where('created_at < ?', due_date).count
+      conference.registrations.where('created_at < ?', due_date).count
     when Target.units[:program_minutes]
-      numerator = conference.current_program_minutes
+      conference.current_program_minutes
     else
-      numerator = 0
+      0
     end
+
     (100 * numerator / target_count).to_s
   end