According to the manual, the recommend minimum diameter is 180mm.
The diameter of a new stone is 250mm.
So the usable reduction in diameter of a new stone is 250-180=70mm.
A stone with a diameter of 233mm can still be reduced by 233-180=53mm.
The grinding area is proportional to the circumference of the stone.
That area is reduced to (53/70)^2=0,57 times the original.
So the remaining 'value' of the used stone of 233mm is 57% of a new stone.
The diameter of a new stone is 250mm.
So the usable reduction in diameter of a new stone is 250-180=70mm.
A stone with a diameter of 233mm can still be reduced by 233-180=53mm.
The grinding area is proportional to the circumference of the stone.
That area is reduced to (53/70)^2=0,57 times the original.
So the remaining 'value' of the used stone of 233mm is 57% of a new stone.