So the typical 90 degree bent arm fuel float is notoriously inaccurate because of the way the tank is set in the frunk. As you discovered a full tank reads at a 1/2 tank and it won't move for a while then it takes off and seems to empty rather quickly. You can remove the sender/float assembly, bend the arm a little more and reinsert the float assembly so that it reads closer to full. Be careful though with the amount of fuel in the tank so it doesn't spill out if it's full. It might actually be easier to empty the tank and put in 2-3 gallons and get it to read empty. At least that way when you have 2-3 gallons left it will read empty but with a 2-3 gallon reserve and you'll want to go get fuel. You can do this with whatever amount of fuel makes you comfortable.