How do you fit a square board into a rounded space. Well with many trips to the saw, or….
Use a template. By tightly taping heavy paper in the very same spot the new board will be fit into, you can “dry run” and get the fit right. Run the edge of a sharp knife perpendicular down the edge of all adjoining boards.
Sometimes I’ll transcribe the paper template onto a temporary 1/8 plywood template if the cuts are more involved. Once saw cuts are made, use coarse sandpaper on a small belt sander to fair it into final shape. Check plywood template for fit and if it looks mint, now use the sharp knife to trace a scratch line onto the wood flooring board. Hold to this scratch line and it’ll be just right.