Cadillac 2005 XLR Owner's Manual page 338

3. Fill the coolant surge tank with the proper mixture
until the level inside the surge tank stabilizes at the
FULL COLD mark on the front of the surge tank.
6-30
4. With the coolant surge tank pressure cap off, start
the engine and let it run until you can feel the upper
radiator hose getting hot. Watch out for the engine
cooling fan.
By this time, the coolant level inside the coolant
surge tank may be lower. If the level is lower, add
more of the proper mixture to the coolant surge tank
until the level stabilizes at the FULL COLD mark
on the coolant surge tank.