Planning fuel requirements has become increasingly difficult.
We carry two basic kinds of fuel oil, bunker oil and diesel. Of the two basic types each can be either regular sulfur or low sulfur. So the ship can have up to four different types of fuel: The regular bunker is IFO-380 is up to 4.5 % sulfur, LSFO is less then 1.5% sulfur. Of the diesel there is MDO or marine diesel oil and there is MGO or marine gas oil. LSMGO must burned in certain areas of the world and must be less then 0.1 % sulfur.
Each of these fuel must be kept in separate tanks. The diesel is not a problem, we have two tanks, 50 tons each, we burn about 30 tons a month. As to heavy oil, the ship has 3 sets of tanks for a total of 6, they are normally used in pairs. Total capacity at 85% is about 2630 tons. Of this we burn 50-60 tons a day.
The heavy oil must be loaded into an empty tank, no " mixing" is done because of compatibility issues. On arrival into the bunker port, which ever tank is planned on being used, must be empty. In addition having extra fuel on board increases displacement which in turn increases fuel consumption. Another factor is price. If fuel is available at a bunker port further along in the voyage often the ship is asked to take minimum bunkers.
So there are several reasons not too have too much fuel on board. All this means is that it is important to be able to plan fuel usage. I compute fuel usage two ways, in port by tons/hour and at sea by miles/ton. When I receive a schedule I create a spread sheet with all the distances between ports and an estimated time in each port. I can then easily compute fuel remaining on board (ROB) for each port. Then knowing the total ROB I create a second sheet that shows ROB by tank. This allows me to plan which tanks are available at each bunker port. Every day at noon, on a third sheet, I enter the distance traveled and fuel consumed for the day and compute the miles/ton. The average is about 7.5 to 8.5 miles per ton at full speed depending on weather.
This all sound like a lot of work but once its set up it can be done quickly. The trick is getting the distances correct. As a check knowing the number of days, which can be gotten off the schedule and the daily consumption can be a way of making sure you're in the ballpark.
In addition to avoiding carrying excess fuel there is one more thing, don't run out. As to margin it depends on route, weather, season and so on but we typically plan to arrive in port with 2-4 days fuel on board. Some like less, some like more.