That was my plan also, but the internet on the ship was poor and spotty, with 2 full days of no internet. I didn’t want to count on being able to proctor, so I made my appointment about a week ahead of our debarkation day. The test result came back in less than 20 minutes. I’m glad I did it that way. If you wait till the day before to make your appointment, it’s possible there won’t be any time spots that fit with your flight schedule. In that case you would have to join the long line of people without appointments. To me the 30 euros I paid to book the test was peace of mind.