I am testing with a full gas tank. I just find it curious that this time it took 10 gallons and last time it took 8 gallons, the rest of the times have also been inconsistent.
OK so If I understand now... The tank is full, The gauge says approx half tank and approx 23 ohm.
Your gauge is good. My best assumption is your sender has fuel in its float and will not read properly. If the sender could/would go to 10 ohms it would take the gauge to full.
So next step , buy a new sender. Drive all the gas out of it. Install new sender.
Sure you could probably buy a float and a seal kit. If it turns out to be a bad sender, a new sender comes with float and seal kit.
as for 10 gallons then 8 gallons.. That's typical. Its a range indicator, never was a gallon for gallon instrument.