How the numbers are worked out
Cash & carry (buy cash, sell future) uses the future's bid minus the cash ask. Reverse (sell cash, buy future) uses the cash bid minus the future's ask. Gap % is measured against that cash price.
Expense is the statutory charges for the full round trip — buying and selling the shares (delivery) and the future, both closed before expiry — as a % of the cash price. Brokerage and DP charges are not included. Net gap = gross gap − expense.
For information only — not investment advice. Check every price with your broker before you trade.