Your boat is setup with the fuel tank to be level when the boat is on the water, not on the trailer. That said, your fuel gauge will read it's true reading when the boat is sitting at rest in the water. Also, your fuel vent is also placed in the tank to be at the highest point when the boat is in the water. This means the tube and/or the part of the tank it is tapped into can pocket with fuel. Eventually as your are filling with fuel, the air pocket in the tank can and will overcome this fuel pocket pressure and whaaaalaahh... you have fuel pushed out the vent all over the side of your boat.
Most of our boats are set up so the nose is down when towing compared to sitting at rest in the water.
My suggestion, if you want to get incredibly anal about it; would be to get a couple of $5 motorhome levels at Wal*Mart. Next time you take the boat out, stick one of the levels inside the boat inside of a storage compartment, while the boat is at rest, and stick it on as level at that time.
Then when you get back on the trailer, you can use your tongue wheel to adjust your trailer to know when the boat is at exactly the same level. Take the second level in the package, and stick it on the trailer, so you can always match the same angle with the tongue wheel at anytime.
Also, remember your tank is NOT square on the bottom. It's V shaped! So 1/2 full on the gauge is not 1/2 full according to the tank. If you get down to approaching 1/4 tank, you need to get fuel, very soon.