Set the barometer in a stable place. Write down the number the gauge reads. This is the gauge pressure.
Write down in your calculations that the atmospheric pressure is constant at 760 mmHg (or millimeters of mercury). One atmosphere is equal to 760 mm of mercury, which is the approximate mean sea level pressure.
Add the gauge pressure to the atmospheric pressure. This is the absolute barometric pressure.
Apply the barometric formula. Multiply the number for absolute barometric pressure found in Step 3 by the e constant, which is about 2.71 (also found on a calculator), to the exponent of -mgh/kT. M stands for the mass of the molecule, g is the gravitational constant (about 10), h is the change in height from sea level to that level of altitude, k is the Boltzmann's constant (1.3806503 --- 10^-23 m^2 kg s^-2 K^-1) and t is for temperature.