resting idle in neutral is 18 on mine. 19 on axel's starion which runs a vr4 motor.
I've been through 3 boost gauges, an autogauge, an autometer and a speco (which I liked the best and was coincidently cheapest)... they all read 18, except hte autogauge which liked to round things off... ie: it was at 20.
If you're coasting (ie: driving) it'll be around 22 in 5th or so.