Forget the BBQ lid thermometer and get a Redichek:
... or similar dual probe thermometer.
Bimetal strip thermometers on BBQ lids are not very accurate according to Alton Brown: "Bimetal coil thermometers are about as accurate as a sniper scope on a nerf gun."
http://amazingribs.com/BBQ_buyers_guide/thermometer_buying_guide.html