Getting that reduction requires a credit of 0.284% of the property’s value. But in May, the Board of Estimates approved a credit of 2.048% — the target average rate, and almost 10 times higher than the credit should be. The mistake came from a clerical error in a memo submitted by the city’s finance department for the board’s agenda. It was not connected to a ransomware attack on city computers in early May, a spokesman for the mayor said.