‘97 Baja 272 Islander

The fuel gauge has never shown completely full or completely empty and just shows 3/4 full down to 1/4 left. How do you troubleshoot such a thing?

pull the sender and see what it does when you move the float and see how float movement relates to the tank

Not sure what guage and sender you have.
My Gaffrig/Livorsi sender has 2 small screws for adjusting the Full and Empty settings. It takes some trial and error to get it set right

I believe it’s the original Faria gauge and sender that came with the boat. It looks like I’ll have to pull up the floor to get to the sender.

Baja you can do a quick test before pulling the floor. Most marine senders use 33 ohms to 240 ohms of resistance to determine fuel level. Check what the resistance is at full tank, and at empty. That will determine if it's your gauge or sender.