The sender itself is adjustable. You do need to remove it to do it, so you'll want to do it with 1/3 of a tank of less.
But, before doing that, it will also affect the upper end of the scale, so when full, were does it read ?
Didn't know that, will have to have a look when I get the time, when full it does actually read full so it's at least accurate at one end of the scale!
This is a sender from a Moke Fuel Tank, but for what counts here, the Mini ones are the same
The slot in that shaft allows for adjustment between the resistor and the arm, so while it is an adjustment, it does change the readings at both ends of the scales, ie, if it's reading high on the empty end now and this is adjusted, while you can set it to read empty when empty, it may not quite read full when full, but a little under.
The Smiths Gauges also had adjustments within them and at each end of their scales. If you were patient enough you can really 'tune' these in to read spot on right across the whole range - as long as the sender was in good condition. Sadly, the Nippon Gauges doen't have this adjustment within them.