10% of a 90db reading is 9db. that is pretty big if you ask me.
No.
If the accuracy is +/-10% then this is equivalent to +/- 1 db SPL at 90 db SPL.
Decibels are a logarithmic representation of a signal.
SPL = 20 Log (Actual Signal Reading/Reference Signal Reading)
Reference signal level was set by Bell Labs as the quietest sound the average person can detect at 1 Khz (defines 0 db SPL)