I don't think that the Japanese were concerned about the usefulness of Portugese Timor as a base for their aircraft in 1942. Their problem was that the Allies had already occupied it, so they needed to take it. That well known reliable source Wikipedia has:
Up to this point, the government of Portugal had declined to co-operate with the Allies, relying on its claim of neutrality and plans to send an 800-strong force from Mozambique to defend the territory in the event of any Japanese invasion. However, this refusal left the Allied flank severely exposed, and a 400-man combined Dutch-Australian force subsequently occupied Portuguese Timor on 17 December. In response, the Portuguese Prime-Minister, António de Oliveira Salazar, protested to the Allied governments, while the governor of Portuguese Timor declared himself a prisoner in order to preserve the appearance of neutrality. No resistance was offered by the small Portuguese garrison however, and the local authorities tacitly co-operated, while the population itself generally welcomed the Allied force.
Going back to the original question, the critical range was that of the A6M2. This operated with some difficulty (i.e cruising at low speed with a very lean mixture) from Tainan to Manila, which is 941 km = 585 miles. That is the maximum effective range. Operations from Rabaul to Guadalcanal were further but it was necessary to retain the drop tank in combat, which may have been why the A6M2 did worse against the Wildcats there than against Hurricanes over Ceylon for example.