Mixture is the excess amount of oxidizer over that used for stochiometric reaction. A mixture of one means only oxidizer is used, a mixture of zero means the reaction is stochiometric and a mixture of minus one means only fuel is used. Excess oxidizer is sometimes used when high thrust is needed when, as is usually the case, the oxidizer is denser than the fuel. With hydrogen rich fuel, excess fuel leads to lower combustion temperatures. Hydrogen rich fuel, about 0.02, also generally leads to the highest exhaust velocity. When a rocket liftoff from the ground, they can change their oxidizer mixture to have high thrust on takeoff, then high exhaust velocity partway into the flight when the tanks are partly empty and it is necessary to throttle back. Excess fuel also leads to less oxidation of the engine. The mixture is used to determine the molar ratio of the propellant and reactants which is in turn used to calculate the amount of unreacted fuel.

fuel molar ratio = 1.0 - mixture
mixture oxidizer mix = 1.0 + mixture

unreacted fuel = 0.0
if ( fuel molar ratio > mixture oxidizer mix ) {
    unreacted fuel = fuel molar ratio - mixture oxidizer mix
}

If the combustion isn't ideal:

if ( combustion efficiency < 1.0 ) {

Then the quadratic equation is used to determine the part of each reactant that combines chemically:

    ratio = 9999999.9
    if ( fuel molar ratio != 0.0 ) {
    ratio = mixture oxidizer mix / fuel molar ratio
    }
    unreactability = 1.0 / ( 1.0 - combustion efficiency )
    a = 2 * unreactability * unreactability
    b = a * ( ratio - 1.0 )
    c = ratio + 1.0
    unreacted = 0.5 / a * ( sqrt( b * b + 4.0 * a * c ) - b )
    unreacted fuel = fuel molar ratio * unreacted
}
 
 

This is used in bipropellantrocket, tripropellant rocket, pumped rocket and rocket cost.
 
  Rocket