Started using the last digit of my restaurant bill as a checksum, cause I've had the tip amount changed on me too many times now Decide your rough desired tip, calculate your tentative total, then add up the digits before the decimal point and use that as the number of cents

Total pre-tip bill: $15.90 Rough tip: $3.00 Pre-checksum total: $18.90 1+8 = 9 Final total: $18.99 Now if someone changes the tip to $4 or $5, the checksum fails.