All calculations in chemistry concerning amounts of reactants and products start with a fully balanced chemical equatiiom.
The white external fuel tanks used in NASA’s STS-1 and STS-2 were abandoned in favour of unpainted tanks. Tests discovered that paint was not necessary for successful launches and the weight savings allowed for a greated payload (about 272Kg).
There is no point in saving weight in the form of white paint if you carry more hydrogen or oxygen than you need. How do you ensure that you have just the right number of hydrogen and oxygen molecules to react with none left over?
The starting point is always a fully balanced chemical equation. If you have difficulty following how chemists do this type of calculation watch the chemistry for dummies/numpties video.