Bathroom Floor Transition under door that hasn't been installed
Transitions should fall directly under the door when closed, so when the door is closed it is hidden. Most jobs require installing the floor while doors are hung and we use a door jamb saw to make the cut.
If you lay the flooring first and are off as little as 1/4&quot; to me, it's quite noticeable after hanging the door and something I would not do to a paying customer.
If door is in place. You simply take a piece of the flooring, flip it upside down and butt it against the jamb and use the saw to make the cut.  Make sure to leave enough room for transition strip as provided by manufacturer.

Here is a link to the saw. Home Depot Jamb Saw
And a video of sawing a door jam. https://www.youtube.com/watch?v=apqULE7t45U