Hello I'm confused about where to start. Everywhere I have read says to start on your longest straightest wall perpendicular to the floor joist. My room is very simple roughly 21x12 with the 21 running perpendicular to the joist. I have a doorway just about halfway on the 21 side and the other 21 is an exterior wall.As you come in the room on the right 12 wall in the middle there is a 5x3 closet. So basically a rectangle room. My problem is where to start at the doorway or the 21 exterior wall. If I start at the doorway the jamb is 5 1/2 inches to the hallway floor which is a different floor but same height all together so I would have to put a transition strip there anyways. The flooring is 3 1/4 inches so do I do a lot of measuring so the 2nd board in the doorway matches the 1st row when it comes out. Or do I start from the exterior wall and work towards the doorway. Also I thought if I started at the doorway inside the room I could just put in 2 pieces for the jamb and cut the one under the transition strip but then the 1st one from the room into the jamb will have the groove side not the tongue to nail to and I would think it will split. Sorry for the long post but I am trying to give as much info as possible. Thanks.

I like to work away from a problem. Is the wall with the doorway where you are going to start all parallel? Fit the doors you start with full boards then work away. No need for a transition strip, just fill in between the jambs, then work your way across the room making a rip along the wall that is top-nailed and glued. Having a bundle of inch and a half, and two and a quarter inch flooring helps save material, if you can buy it.

In most cases I try to avoid problems also as they seem to find me without my help. So I'm guessing your saying it's ok to start at the doorway. As for the 1 1/2" and 2 1/4" bundle I don't know what that would do.

If you lay flooring professionally, having other widths of flooring that would save time and materials is worth having the extra inventory. Sometimes it makes laying floor "easy".
For a long rip against a string wall or finishing the floor with a rip that could be made from a different width instead of a 3 1/4 inch wide plank, one of the different widths can be used. A four inch width helps, too, since the extra width would be covered by base board.
For a person laying a single room, just the one size makes the most sense.