The distance to achieve top speed is irrelevant, it is the time lost that matters.
How much time is added to the journey for a single stop (including time for the stop itself, braking and acceleration).
Depends on what you are doing. If you are building track, then distance is what matters. If you are spacing trains out for safety both distance and time matters. If you are planning a schedule time is what matters (unless you are mixing local and express service on one track - see the safety sentence before). If you are planning track than you need to start with ideal schedule and iterate around the rest until you find something that actually works for all considerations.
I just redid the math - to get to 300 km/h needs 4.8km, not 10 that I thought. Faster speeds need more distance or course. Hills also impact the distance needed. You also need to account for same safety distance for a lot of acceleration applications.