Here, Standby duty is calculated as duty time. Every hour (or fraction thereof) of Standby at home counts as half its duration in duty time, Standby at the airport is credited fully and unfactored. The such accumulated duty time of course is followed by the required subsequent rest period as if it was a flying duty.
As all the flight, standby or other duty time is accumulated over the month, a standby duty will only have effects on the pay if the overtime threshold is crossed.
This system is considered acceptable and fair in my company.