Electrics Beetle Fuel Gauge not accurate

Having issues with my 68 Beetle Fuel Gauge.

The gauge doesn't go any higher than half way even when the tank is full.

I've checked the threads on here and Speedy Jim's great page and got the following results.

I've dismantled the screw type float and cleaned all the contacts and tested the resistance using a multimeter.

It reads correct and has a ohm range of 13 to 70 ohms.

The tank is currently almost full and correctly reads 19 ohms.
However the fuel gauge incorrectly reads just under a quarter full..

I then tested the voltage stabilizer using a screwdriver with a test lamp and the light flashes on and off so looks good.

I then tested the fuel gauge. First by grounding the wire which caused the gauge to go up to full. I then tested the resistance between the 2 terminals which was 13.5 ohms.

I then tested the resistance between the 1st terminal and the wire connected to the 2nd terminal as the wire from the gauge to the float was the last link in the chain. The reading for this is 14.2 ohm.

All the above tests I believe should result in the Fuel gauge showing as 3/4 full but it doesn't.

I've also tried different Fuel Gauge, voltage stabiliser and float and all give me the same result of incorrectly showing the tank as quarter full.

Help!