Size for CFDs means number of shares.
Size for spread bets means size of bet
Closing price means underlying market price at 10pm (UK time).
If underlying instrument currency is GBP
Size × closing price × LIBOR +/- 2.5% ÷ 365
Based on one-month LIBOR
If underlying instrument currency is USD
Size × closing price × US LIBOR +/–2.5% ÷ 360
If underlying instrument currecncy is EUR
Size × closing price × EURIBOR +/–2.5% ÷ 360
The formula uses a 365-day divisor for UK, Singapore and South African shares, and a 360-day divisor for shares in other markets.