The Agile-like Outgoing payment is calculated from the day-ahead wholesale prices, Balancing Services Use of System (BSUoS), Generator Distribution Use of System (GDUoS) and Transmission Network Use Of System (TNUoS) per UK region.
The formal calculation is:
Agile Export rate = M * APX + B + C
Where: M is a multiplier to include line losses, imbalance, trading charges, Octopus margin
APX is the APX day-ahead 30-minute wholesale price converted to p/kWh
B is the time of day independent benefits and is based on BSUoS and GDUoS
C is the time of day dependent (peak) benefits and is based on TNUoS.  (C only applies from 4-7pm)
Check out sample half-hourly Outgoing prices by region for the whole of 2018 here
The table below shows the set of regional coefficients for M, B & C.  APX is published for the day-ahead at around 4.00pm for the next 11.00pm to 11.00pm slot.