FWIW, this is what is said about turn
indicator (not
coordinator) errors in my (CAA-flavour) CPL course notes, as produced by the sadly now defunct Professional Pilot Study Centre:
"The rate of turn indicated will only be correct when the aircraft is flying at the true airspeed for which the instrument has been calibrated. Providing that the actual TAS is within 100kts of the calibrated TAS the error in indicated rate of turn should not exceed plus or minus 5% of the actual rate of turn. The instrument over-reads if TAS is higher than the calibrated airspeed."