I am sure that there is a lot of experience in this Group that the "new-to-Amel" Super Maramus and 54 owners would like to hear regarding onboard fuel. When I was new to this Group, I could not get experienced answers on how much is enough fuel.
I bought 10 each 18.9 liter jerry jugs and filled them for each ocean crossing. They added about 300nm to my about 1,000nm range on the Super Maramu (600 liter tank). BTW, the 54 fuel tank is 900 liters and should provide a minimum of 1,300 miles while motoring without carrying extra fuel.
My experience around the world is that we never touched the 10 jugs (189 extra liters) and the most that we used on any passage was 400 liters in an isolated case with almost no wind between Panama and the Galapagos. Had we been more experienced, we may have picked a better departure date. During our two 3,000 passages we used about 250 liters each.
I hope this helps you make your plans.