You need to check the gauge while properly connected with a IVR/ limiter, and with known test resistances. That fuel gauge with each resistance should read the same as the temp gauge. If you have a Ralleye cluster, the oil gauge should also read the same as the other two, or at least "within reason."
You say it "all checks out." This is a bullshit statement. We can't sit here and see what you've done, or know what you've actually "checked." If it all checks out LOLOL, then it should work, right?
I would not rely heavily on the "1.5v" test. That will give you some idea, that is all.