I'm planning on installing hardwood in my 2nd floor, and thought I would run by my idea of how to start. At the top of the stairs is a perpendicular hallway which branches off 2 rooms to the right, and the master to the left, with the bathroom at the end. I'm planning on starting the hardwood in the middle of the hallway, as a base point. I'll measure to the middle at each end and snap a line. I'll then measure to the back wall of the two rooms, and hopefully it will be the same. After installing in that direction, I will have a double sided tongue which i will secure into the beginning to reverse directions for the master.
I think this will work, but i'm open to suggestions.
What do you think?

That will work. Just triple check your line to be sure it is straight. Use some spline/slip tongue to change directions.

Do the set up as you describe, but I would then measure off the line over to one of the hall walls and start there. much easier to turn around in a couple of doorways, than down the entire length of the hall.