My dehumidifier has an error margin of +/- 5% in the range 30%-90%
In theory, "+/- 5% in range of 30%-90%" means that...


if you set the humidistat dial to 30%, it might actually be set to anywhere from 25% to 35%
if you set the humidistat dial to 90%, it might actually be set to anywhere from 85% to 95%


In reality, consumer humidistats generally aren't very precise (e.g. relatively less precise than most thermostats). Unless you have an absolute need for a precise humidity level, I'd just set it to what feels good and not worry too much about the absolute humidity level.