Jean Marie de Montague is listed as a midseason bloomer by the American Rhododendron Society. So, in your case mid season would be around the second week of May in a typical year if Jean Marie consistently blooms at this time.

The seasonal bloom designations are, however, relative and will vary considerably from year to year and, of course, from area to area. The way you're doing it - using a particular variety as a marker plant - is the best way to get a reasonably accurate indication of when other varieties will bloom.

Yes, that's exactly what I was trying to do. Using Jean Montague as a marker. I guess it would be difficult for the ARS to put in exact months because like you say it would be depend on location. That helps to know. Thanks.