"A missionary in Mozambique does not keep records of her actual meal expenses as she was told that there was a basic “formula” used in deducting food expenses. Can you provide any information regarding this “formula” for calculating food expenses?"
First, most meals for missionaries are not deductible. According to IRS Publication 463, meals are only deductible when it is necessary for you to stop for substantial sleep or rest to properly perform your duties while traveling away from home on business. Business-related entertainment meals are also deductible. Since a missionary's home is his or her place of ministry in the foreign country, he or she is rarely traveling away from home on business. The exception may be short ministry-related trips within the country.
For these cases, Publication 463 describes a standard meal allowance. The IRS says, "If you use the standard meal allowance, you still must keep records to prove the time, place, and business purpose of your travel."
IRS Publication 463
The above link may be followed to find details for standard meal allowances in foreign countries (additional links are impeded in the Publication--page 6 in the February 4, 2009, printing). The Publication also provides details regarding partial days.