If you repeated the experiment with 10 mL less hot water, the molar heat of fusion of water would be expected to be the same.
What is molar heat of fusion?
The molar heat of fusion is the amount of heat energy required to melt one mole of a solid substance to a liquid at its melting point. It is typically measured in units of joules per mole (J/mol).
In the experiment you described, you are measuring the molar heat of fusion of water. You have a certain amount of water at a certain temperature, and you are adding hot water to it until it all melts. You are then measuring the amount of heat energy that you added to the water.
If you repeated the experiment with 10 mL less hot water, the molar heat of fusion would be expected to be the same. This is because the molar heat of fusion is a property of the substance being melted, and it does not depend on the amount of substance being melted.