You can tap into the brake booster line (as per attached pic), this will give you a much more stable reading on your gauge, I believe the line is 3/8", I was running my gauge off here for a while, but you need to be mindful of heat though as its sitting around that whole exhaust / turbo area, there is a large grommet located in the firewall not far from this location that you can run the line through, it enters the cabin just behind the glove box.
Nearly all men can stand adversity. But if you want to test a man's character - give him power.
Abraham Lincoln