Your sending unit should read 240 ohms of resistance on an empty tank, and 33 ohms of resistance with the tank full. Disconnect the sender from the gauge, and use the resistance scale of your VOM to measure the resistance right at the sender. It should be about right for the amount of fuel you have.
1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida