As I am typing the question I know there is no easy answer, but I thought I'd solicit suggestions anyway. I have had to change my plans because of a family reunion that has been planned for the middle of the summer. I was planning to leave the dates open and leave Oregon when the passes opened... arrive on the East Coast when I arrived. But now the family has planned a reunion and I have to be in Bozeman Montana on a certain date the end of July. So I am changing plans and will leave Yorktown, VA in May and travel E-W. But what date do I leave?
I am asking for any ideas on how to plan a departure date. This is my first long tour - longer than a week so I'm not positive how many miles a day I will travel or how many rest days I'll need. I could estimate very conservatively but I don't want to arrive in Montana a week early for the reunion

I certainly don't want to arrive late. My first thought was to just count the miles and divide by 50 miles per day and add one rest day per week.
If any practiced travelers have wise suggestions I'd appreciate your thoughts...