Class E with a floor of 1,200 feet AGL is shown with the "fuzzy" side of the blue. And sometimes Class E has a floor higher than 1,200 feet. They will print the floor of the Class E altitude in the "fuzzy" side of the blue. I know the Class E with a floor of 1,200 feet AGL is usually the floor of an airway. It goes from 1,200 AGL to 18,000 MSL. But if there is a designated specific altitude higher than 1,200 AGL for Class E airspace floor covering a federal airway, at what altitude does the Federal Airway start?
Likely at the floor of the Class E airspace. A Federal Airway coinciding with a Class E floor higher than 1200' AGL would normally only exist in mountainous terrain.
A definitive example could be found by looking in the FAA's "Airspace Designations And Reporting Points" document, available on-line.