We have one fuel tank of 200 gallons below sea level and under the cabin sole (middle of the boat). I just pick up a smallish floorboard (middle of the boat, unscrew a 1.5" plug that's just below the sole and drop a stick in the tank. On the bottom of the floorboard I picked up is the inches = gallons chart.

An interesting part of filling the tank is that we can look in the tank as it's being filled. The Admiral likes seeing the fuel level and watching the fuel as it's being filled and I have to admit,, I like the simplicity of the system as I can never get a wrong reading of how much fuel is in the tank and by monitoring through the hole in the tank we never worry about a deck overflow.

When I pull the stick out I just hold a paper towel on the bottom for any drips. Not messy at all.