It is recommended to never run your boat lower than 10% of its total fuel tank volume advertised. For example, if you have a 140L tank you should never run it lower than 126L. The advertised tank sizing's do not allow for situations such as boats being filled when not perfectly level which will not allow the fuel tank to be filled completely. Pitch and roll also effect/ restrict the fuel being able to be picked up when the tank level gets low. You should always fuel up on the way TO the ramp and NEVER store a boat FULL of fuel.
Calculating trip fuel is best done by resetting the trip fuel on the engine management gauge at the start if a trip after fueling up which will let you know exactly how many litres have been used on a trip. Get in touch with your local dealer to talk you through how to do this is you are unsure. The sender unit reading will give you an idea how much fuel there is in the tank, but as the boat is not always level there will be variation in the reading.