This thing has me pretty frustrated. I kinda did what you describe but instead of running it to the battery, I ran it to from the pin on the gauge to the frame. When I turned the motor over the gauge went all the way over to the 4oclock position. Then, I took the original ground, tighten up the connector, cleaned the pin. Now when I turn the key to the on position the gauge is where normal operating temp has always been but if I turn the motor over the gauge goes back up over the red line again.
I thought I was on to something with this ground idea because I noticed in the last few drives the speedometer fluctuates, which it never has, and thought maybe the gauges shared a ground and it was loose. Now, I have no idea.
Stupid Land Rover.