I had a similar situation where I had a spot in the middle of the room where I needed the corner of a board to hit exactly (link: Prep & Install Salvaged Heart Pine Floor
). The solution I used was to lay that board first and then rip down some scrap wood to insert into the groove in that first board to make it double-tongued. I then laid the floor in both directions from that starter board. If you're certain you want/need a full board for the bullnose (which, if nothing else might be important just do you don't end up with a 1" bullnose course) I'd do the same in your situation.