I couldn't swear to it, having never done it with CMS 20, but I would suspect a good compensating developer like DiXactol if you are using 120, or 510 Pyro, would be the way to tame a high contrast film. It certainly works with Pan F, and means the contrast range is kept well in check without producing flat negatives. Both developers also provide a lot of exposure leeway while you are nailing down your exact times.