Actually, the entire guage needs filled. The bourdon tube and linkage have much more mass than the needle does so they have to be submerged in the glycerine / silicone fluid. The gauges aren't vented to atmosphere as the glycerine would weep out as it expands, that's why they leave the air pocket so the seals don't pop.
Easiest fix would be to use a non-filled gauge mounted to the fender.
Or of course mark the gauge reading when the engine's hot.
