A quick search found this nugget...
A rule of thumb for the altimeter correction is that the pressure drops about 1 inch of mercury for each 1,000 foot altitude gain. If you're using millibars, the correction is 1 millibar for each 8 meters of altitude gain. These rules of thumb work pretty well for elevations or altitudes of less than a two or three thousand feet.
For the units to make sense...
In the U.S., air pressure at the surface is reported in inches of mercury while air pressure aloft is reported in millibars, also known as hectopascals (hPa). Scientists, however, generally use pressures in hectopascals.
In the rest of the world, measurements are usually given in hectopascals although you will sometimes see them in centimeters of mercury, especially on older barometers.
The term "hectopascals" is replacing the term "millibars." The hectopascal is a direct measure of pressure, like pounds per square inch, but in the metric system. Since the measurement is in the metric system, 1,000 millibars equal one bar. A bar is a force of 100,000 Newtons acting on a square meter, which is too large a unit to be a conveinent measure of Earth's air pressure. Inches of mercury measure how high the pressure pushes the mercury in a barometer.
To convert between inches of mercury and millibars, one millibar is equal to 0.02953 inches of mercury. The El Paso, Texas, National Weather Service Office has a weather calculator posted on the web that can be used to make the conversion.
Centimeters of mercury can be translated into inches of mercury by dividing the number of centimeters by 2.54.
The use of direct pressure measurements goes back to the late 19th century when the great Norwegian meteorologist Vilhelm Bjerknes, a leader in making meteorology a mathematical science, urged weather services to use direct pressure measurements because they can be used in the formulas that describe the weather.
A sidelight: In the International System (SI) of measurements, the unit of pressure is the Pascal, named after Blaise Pascal, the 17th century scientist who made important discoveries about air pressure. The standard atmospheric pressure at the Earth's surface of 1013.25 millibars is equal to 101,325 Pascals. To avoid large numbers, air pressure is reported in hectoPascals, which are the same as millibars. In many nations, you are now likely to hear reports such as, "air pressure, 1020.0 hectoPascals." This is the same as 1020.0 millibars
You can do the math.
Marc
