I used usual rounding.
But after each cut next 500g I measure not from ideal cut line, but from real cut line, taking into account accuracy of cutting machine.

tricky:
needVolume = 500.0;

while (needVolume < <total volume> )
{
   <find y>
   iy = round(y)
  recalc volume, from 0.. to iy position.(v).
   needVolume =  v + 500.0
}