[Marshall V. Daut]

If the gaskets are installed correctly, you can fill the tank as high as you want. You must follow the correct sequence of parts replacement and use the thin brass slip ring AFTER the gauge has been inserted into the tank. That ring acts as a spacer and friction barrier when the two bezel nuts are tightened, especially the large outer one. The gauge is not as likely to spin as the bezel nut is tightened if at least one brass ring is between the gauge and the outer bezel. It's your call whether to use neoprene or cork gaskets. I used neoprene for years without any problems, but with the newer gasoline mixes containing ethanol and Lord knows what else, I'm reading that the neoprene gaskets are deteriorating, causing leakage. I've gone back to cork and have no leaking gauges in years.
By the way, buy the expensive brass bezel tools, not the cheapie pot metal or aluminum ones. Because the pot metal and aluminum tools are not precisely made and fit around the outer bezel loosely, slippage will occur when tightening. Also, the wimpy handle has a tendency to break if too much pressure is applied. You want to get both bezels as tight as you can without breaking anything. And wear heavy gloves when tightening the bezel nuts because if the outer bezel tool should slip off, you WILL cut your knuckles.
Marshall