ShortanswerisnoIdon'tknow.
Regular AJ answer;
The stock gauge seems to be electrically damped. If you ground the sender wire,you will see how slowly the needle moves. I don't think you can or would even want to,do anything about that. But if the reading is wrong and the sender works properly, I sorta calibrated mine by bending the float arm till it was close enough. But one thing that caught my attention is your EFI return line; if it is spewing onto or near the float that could mess up the reading. Also, I found that on mine, the slipyoke was easy to mess up during calibration, so be careful. I have heard of installing a resistor in parallel with the sender to help it to read at one end or the other, but I thought that was hokey. I just calibrated mine to read empty when there are about 5 liters left in the tank, sorta like a reserve indicator, and let the full be where it will be,lol.
The gauge wants a 5.0 volt power-supply . If your dash-regulator is supplying any other voltage, it will read wrong all the time. Either hi with higher voltage or lo with lower voltage. If the supply voltage fluctuates due to a problem in the charge circuit,that will also mess your gauge up. Your IVR may be the type that steps the charge voltage down as a percentage. For instance 5/13.2 = about 38%. if your charging system is putting out 15 volts, then 38% of 15 =5.7 volts so the gauge goes hi. And if the charging voltage drops to 12.0,then 38%=4.6 and the gauge reads low.So if the charging system voltage is fluctuating, then the dash gauge would mimic it.
My IVR is a choke; it only lets 5.0 volts thru, no matter what it is supplied with. That keeps the temperature gauge,which also runs off it, rock solid, so it doesn't scare the .......out of me. That oem temp gauge seems to only work during warm up tho. After that it seems to get stuck. In all the years I've driven that car, minus the first few months, I've never seen the factory gauge move from it's "normal" position. I hate to admit it but I never look at it any more.