I don't know how low the base will be. Now we are getting somewhere! The problem is crucial 'details' like this are unknowns. It depends who you listen to. ASA seem to think 12500', or several different figures around this altitude, in reality you will have to have no higher than 7500' as a base for the bulk of the twins to operate in (unless, of course, the plan is to provide almost no service to the industry), and Dick mentions 1200'. Everybody involved has got a different answer/desire. The devil is in the detail.