Calculation of accuracy for digital multimeters

I think this is a good question. Digital meter manufacturers use these accuracy statements without explanation. My best guess is if they are stating 2% rdg plus/minus 5 counts, they mean plus or minus the sum of 2% of the reading and five times the least significant displayed digit. For example a 3 1/2 digit meter will display 0 to 1999 or 2000 counts. If a reading was 20, the accuracy would be 2% of 20 (.4) plus 5 or 5.4 rounded up to 6. So the reading would be 20 plus or minus 6. If the reading was 100, the accuracy would be plus or minus 7. If the reading was 1000, the accuracy would be plus or minus 25. "d" probably means the same thing as count. It would be nice to know for sure what this nomenclature means.

This should be done by metrology that have very accurate equipment to check calibration.