currency futures and basis Casasophia(june 11) Polytot(june 2004)

Its about the basis adjustment we make on the future spot rate to determine the lock in futures price. Sometimes the future spot rate is reduced by the basis and some times added on the future spot to determine the futures price. In june 2011 question casosophia co, the basis is being deducted from the 5-month expiry futures(1.3698) and if you see in June 2004 question Polytot plc the basis (0.0078) has been added to the futures (1.5275) to determine the lock in futures price.
When do we add the basis and when to deduct from the future spot rate??

Please help