My 2005 DB9 comes up to temp the same every drive. I let the needle get a "tick" off the bottom of the gauge and bring the oil pressure up prior to taking her out. The temp comes up to half way and sits. A 40 minute joy ride or a 4 hour road trip, it doesn't matter. Not 49 or 51%, dead on half way. Every time I return to the garage I open the hood to assist cooling. The engine temp feel seems to be the same each time in my 2 years of ownership. Is this normal? Is there a way to test the gauge?

 

Plug in a code reader and find the coolant temperature, I'm sure it'll fluctuate whilst the needle stays still.

Other performance cars I own do the same but when you log the temperature from the ECU it becomes apparent that small changes in temperature move the needle a lot initially until you get into the operating range, say 80-100 degrees. During this range the temperature fluctuations dont move the needle, but get out of this range and it'll move quickly again.

Essentially it isn't a linear scale.

 

I have an 05 as well. My temp gauge worked the same way you are describing when I first got the car. I've changed the thermostat for the one that comes in the Vanquish, Virage, and 2013+ DB9. It opens 8 degrees celsius sooner if i'm remembering correctly. I also did a coolant flush and added Water Wetter to the new coolant. Percentage wise, the needle will fluctuate between about 45% and 52% now after driving when fully warmed up. That said, the gauge is probably working correctly but this could be a solution if you want it to run cooler.