I am wondering over what timescale this pressure trend is calculated (three hourly, hourly, minute to minute), and what change in pressure is required to define rising or falling?
Many thanks.

The pressure three hours past subtracted from the current pressure. If the difference is more than one it is moving. Negative is falling. Positive is raising.

Not sure who to notify but there is a small unit error for the acceleration due to gravity, g, on that page under the Sea Level Pressure. The negative sign should be removed from the seconds squared unit since it is already in the denominator. The way it is written it means meters seconds squared rather than meters per second squared.