1) The barometric pressure as reported by a nearby station. Near both in time and distance. This value will have been adjusted to a sea level value. You can find archived data at Weather Underground. If that isn't available just use the default value provided.
2) Launch site altitude. You can find this on topgraphic maps or Google Earth.
Once you have entered the data, click on the "Calculate" button to see the results.
(Note: This calculator switches equations at 36,152' as required by the NASA model so it is valid throughout the sensor range.)
You can improve the results from the Flight Analyzer program if you replace the offset. The default calibration file is named "prodata.cal". Use your favorite text editor to replace the offbp value with the offset shown above multiplied by 2.7. It will not be perfect but it will be better than the default value.