To what time shall I book my departure from Padang, after the boat trip?

Usually the boat arrives back in Padang before lunchtime on the last day of the trip, and the guests can fly from Padang to Jakarta or to Kuala Lumpur during that same day, after 4 p.m.
Nevertheless we advise you to book your flight from Padang for the day after the end of the trip (staying overnight in Padang) because we are never sure about the time of arrival back in Padang. It depends on the weather/sea conditions during the crossing back to Padang, and in case of rough seas there’s a (small) possibility of delay, and consequently a (small) risk of missing your flight.
It’s up to you to take the risk and book your flight from Padang for the day of arrival (after 4 p.m.), or play safer and book your flight for next morning, sleeping that night in Padang.