The Central Isles of Matar were almost straight south of Tenger. But Ackster wasn't starting his journey across the sea from Tenger. And he only had an approximate understanding of where Bo's island was in correlation to Tenger and Matar.
But that wasn't a problem for Ackster and his boat, which only had a similar outer appearance to his other sailboat. The boat's specs were in an entirely different league.
Now that Ackster had mana, he could use the boat's functions, such as the magical navigation tool that worked eerily similar to a satellite map or a GPS. If he had enough mana and was willing to use it, he could even propel the boat forward instead of relying on uncertain winds.