A rule of thumb for 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 more details and a formula for your spreadsheet use Google to search the web. Many places have this answer, such as http://www.usatoday.com/weather/wbarocx.htm (scroll down the page).
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