[tsubaki]

Normally it is only possible to make adjustments on the sending unit and not the gauge.
Depending on the type of sending unit you have, you can do the adjustments that you are describing.
If the sending unit is the stem type, usually the only adjustments possible would be for the tank depth.
The usual sending units of the arm type can be manipulated some in order to reflect a know level (as in your case) by bending the arm at some point between the float and the sender itself.
Bending the arm to make the float deeper would adjust the sender toward a more "full tank".
In your situation, it is probable in doing this, the arm may contact the top of the tank when not quite full of fuel.
If it were me, I'd have to find the actual tank size, even go to the point of draining it, refilling it to full, then remove 1/2 of the fuel and then do the adjustments to the sender or the manual portion, whichever is incorrect.
Again, this is making sure the tank is level.