I read your initial post again and then I checked to see how my 200LS works - during the day I don't see any illumination of the odometer or radio buttons - just the channel display is fully illminated, When the automatic headlights go on when its dark out , the odometer and radio buttons illuminate while the channel display ( ie 104.7 or whatever) dims a bit. The only variable dimming of the panel/ radio buttons my vehicle has is when the headlights are on at night. If you are experiencing dim lighting of the radio display during the day, that is symptomatic of the automatic head lights being on on durng the day. This exact scenario happened to me last summer when the connector to head light sensor in the dash came off. I could hardly see the radio display during the day. There are most likely other causes of this, too ( ie dirty sensor cover).
To answer your question about tolerances using a multimeter, if your battery voltage when the car is off for a few minutes reads 12.5 volts DC or thereabouts, then your test voltages when the car is off should be very close to that. Problems with night time excess variable dimming still point to the headlight/dimmer switch or low circuit voltage being at fault.
Good luck.