I have a question about single-level records: are they to be timed from the end of the preceding level/start of the new level or from clicking the box on the screen. Also, how should I ensure my timing is accurate? Right now my only method is to click my timing key on the start and finish of a level, but this is very inaccurate especially when considering level lengths of around 2 seconds.

Either the end of the preceding level or the click of the box is appropriate. To ensure your timing is accurate, I would hit your timing key and click the level simultaneously, and hit the timing key the instant you touch the goal.